Most of the definitions in this glossary have been taken verbatim from Wikipedia. Whilst that site might not be the most authoritative site regarding Buddhism, the definitions provided will suffice for the new Neophyte on his or her journey toward Nibbana. Also there are various definitions included that are not of Theravada origin, I have included these because of their prevalence and most budding Buddhists will come across them at some point in their career.
Each definition is linked to the appropriate Wikipedia article and I am sure that, should the reader so wish, a good Google search will provide more substantial reading matter for those so inclined.
AN – Anguttara Nikaya – The Numerical or ‘Gradual’ Discourses.
DN – Digha Nikaya – The Long Discourses
KN – Khuddaka Nikaya – The Minor Discourses
MN – Majjhima Nikaya – The Middle Length Discourses
SN – Samyutta Nikaya – The Connected Discourses
Arhat – Theravada Buddhism defines arhat (Sanskrit) or arahant (Pali) as “one who is worthy” or as a “perfected person” having attained nirvana. Other Buddhist traditions have used the term for people far advanced along the path of Enlightenment, but who may not have reached full Buddhahood.
The understanding of the concept has changed over the centuries, and varies between different schools of Buddhism and different regions. A range of views on the attainment of arhats existed in the early Buddhist schools. The Sarvāstivāda, Kāśyapīya, Mahāsāṃghika, Ekavyāvahārika, Lokottaravāda, Bahuśrutīya, Prajñaptivāda, and Caitika schools all regarded arhats as imperfect in their attainments compared to buddhas.
Mahayana Buddhist teachings urge followers to take up the path of a bodhisattva, and to not fall back to the level of arhats and śrāvakas. The arhats, or at least the senior arhats, came to be widely regarded[by whom?] as “moving beyond the state of personal freedom to join the Bodhisattva enterprise in their own way”.
Mahayana Buddhism regarded a group of Eighteen Arhats (with names and personalities) as awaiting the return of the Buddha as Maitreya, and other groupings of 6, 8, 16, 100, and 500 also appear in tradition and Buddhist art, especially in East Asia. They can be seen as the Buddhist equivalents of the Christian saints, apostles or early disciples and leaders of the faith.
Bhikkhu – A bhikkhu (Pali, Sanskrit: bhikṣu, Sinhala: භික්ෂුව) is an ordained male monastic (“monk“) in Buddhism. Male and female monastics (“nun“, bhikkhuni (Sanskrit bhikṣuṇī)) are members of the Buddhist community.
The lives of all Buddhist monastics are governed by a set of rules called the prātimokṣa or pātimokkha. Their lifestyles are shaped to support their spiritual practice: to live a simple and meditative life and attain nirvana.
Bodhisattva – In Buddhism, bodhisattva is the Sanskrit term for anyone who, motivated by great compassion, has generated bodhicitta, which is a spontaneous wish and a compassionate mind to attain buddhahood for the benefit of all sentient beings. Bodhisattvas are a popular subject in Buddhist art.
Brahmins were traditionally responsible for religious rituals in temples, as intermediaries between temple deities and devotees, as well as rite of passage rituals such as solemnising a wedding with hymns and prayers. Theoretically, the Brahmins were the highest ranking of the four social classes. In practice, Indian texts suggest that Brahmins were agriculturalists, warriors, traders and have held a variety of other occupations in India
Buddha – Gautama Buddha (c. 563 BCE/480 BCE – c. 483 BCE/400 BCE), also known as Siddhārtha Gautama [sid̪ːʱɑːrt̪ʰə gəut̪əmə], Shakyamuni Buddha [ɕɑːkjəmun̪i bud̪ːʱə], or simply the Buddha, after the title of Buddha, was an ascetic (śramaṇa) and sage, on whose teachings Buddhism was founded. He is believed to have lived and taught mostly in the eastern part of ancient India sometime between the sixth and fourth centuries BCE.[note 3]
Gautama taught a Middle Way between sensual indulgence and the severe asceticism found in the śramaṇa movement common in his region. He later taught throughout other regions of eastern India such as Magadha and Kosala.
Gautama is the primary figure in Buddhism. He is recognized by Buddhists as an enlightened teacher who attained full Buddhahood, and shared his insights to help sentient beings end rebirth and suffering. Accounts of his life, discourses, and monastic rules are believed by Buddhists to have been summarized after his death and memorized by his followers. Various collections of teachings attributed to him were passed down by oral tradition and first committed to writing about 400 years later.
Dana – Dāna (Devanagari: दान) is a Sanskrit and Pali word that connotes the virtue of generosity, charity or giving of alms in Indian philosophies. It is alternatively transliterated as daana.
In Hinduism, Buddhism, Jainism and Sikhism, dāna is the practice of cultivating generosity. It can take the form of giving to an individual in distress or need. It can also take the form of philanthropic public projects that empower and help many.
According to historical records, dāna is an ancient practice in Indian traditions, tracing back to Vedic traditions.
Mahamudra – Mahāmudrā (Sanskrit, Tibetan: Chagchen, Wylie: phyag chen, contraction of Chagya Chenpo, Wylie: phyag rgya chen po) literally means “great seal” or “great symbol.” It “is a multivalent term of great importance in later Indian Buddhism and Tibetan Buddhism” which “also occurs occasionally in Hindu and East Asian Buddhist esotericism.
The name refers to a body of teachings representing the culmination of all the practices of the Sarma schools of Tibetan Buddhism, who believe it to be the quintessential message of all of their sacred texts. The mudra portion denotes that in an adept’s experience of reality, each phenomenon appears vividly, and the maha portion refers to the fact that it is beyond concept, imagination, and projection
Mahayana – Mahāyāna (Sanskrit for “Great Vehicle”) is one of two (or three, under some classifications) main existing branches of Buddhism and a term for classification of Buddhist philosophies and practice. The Buddhist tradition of Vajrayana is sometimes classified as a part of Mahayana Buddhism, but some scholars may consider it as a different branch altogether.
According to the teachings of Mahāyāna traditions, “Mahāyāna” also refers to the path of the Bodhisattva seeking complete enlightenment for the benefit of all sentient beings, also called “Bodhisattvayāna”, or the “Bodhisattva Vehicle”.[note 1] A bodhisattva who has accomplished this goal is called a samyaksaṃbuddha, or “fully enlightened Buddha”. A samyaksaṃbuddha can establish the Dharma and lead disciples to enlightenment. Mahayana Buddhists teach that enlightenment can be attained in a single lifetime, and this can be accomplished even by a layperson.
In the course of its history, Mahāyāna Buddhism spread from India to various other South, East and Southeast Asian countries such as Bangladesh, Nepal, Bhutan, China, Taiwan, Mongolia, Korea, Japan, Vietnam, Indonesia, Malaysia and Singapore. Major traditions of Mahāyāna Buddhism today include Chan Buddhism, Korean Seon, Japanese Zen, Pure Land Buddhism, and Nichiren Buddhism. It may also include the Vajrayana traditions of Tiantai, Tendai, Shingon Buddhism, and Tibetan Buddhism, which add esoteric teachings to the Mahāyāna tradition.
Metta – Mettā (Pali) or maitrī (Sanskrit) means benevolence, loving-kindness, friendliness, amity,  good will, and active interest in others. It is the first of the four sublime states (Brahmavihāras) and one of the ten pāramīs of the Theravāda school of Buddhism.
The cultivation of benevolence (mettā bhāvanā) is a popular form of meditation in Buddhism. It is a part of the four immeasurables in Brahmavihara (divine abidings) meditation. Metta as ‘compassion meditation’ is often practiced in Asia by broadcast chanting, wherein monks chant for the laity.
The compassion and universal loving-kindness concept of Metta is discussed in the Metta Sutta of Buddhism, and is also found in the ancient and medieval texts of Hinduism and Jainism as Metta or Maitri.
Small sample studies on the potential of loving-kindness meditation approach on patients suggest potential benefits. However, peer reviews question the quality and sample size of these studies, then suggest caution
Nibbana (Nirvana) – Nirvana (Sanskrit, also nirvāṇa; Pali: nibbana, nibbāna ) is the earliest and most common term used to describe the goal of the Buddhist path. The literal meaning is “blowing out” or “quenching.” It is the ultimate spiritual goal in Buddhism and marks the soteriological release from rebirths in saṃsāra. Nirvana is part of the Third Truth on “cessation of dukkha” in the Four Noble Truths, and the summum bonum destination of the Noble Eightfold Path.
Within the Buddhist tradition, this term has commonly been interpreted as the extinction of the “three fires”, or “three poisons”,[note 1] passion, (raga), aversion (dvesha) and ignorance (moha or avidyā). When these fires are extinguished, release from the cycle of rebirth (saṃsāra) is attained.
Nirvana has also been deemed in Buddhism to be identical with anatta (non-self) and sunyata (emptiness) states. In time, with the development of Buddhist doctrine, other interpretations were given, such as the absence of the weaving (vana) of activity of the mind, the elimination of desire, and escape from the woods, cq. the five skandhas or aggregates.
Buddhist scholastic tradition identifies two types of nirvana: sopadhishesa-nirvana (nirvana with a remainder), and parinirvana or anupadhishesa-nirvana (nirvana without remainder, or final nirvana). The founder of Buddhism, the Buddha, is believed to have reached both these states. Nirvana, or the liberation from cycles of rebirth, is the highest aim of the Theravada tradition. In the Mahayana tradition, the highest goal is Buddhahood, in which there is no abiding in Nirvana, but a Buddha continues to take rebirths in the world to help liberate beings from saṃsāra by teaching the Buddhist path.
Nikaya – Nikāya is a Pāḷi word meaning “volume.” It is used like the Sanskrit word āgama to mean “collection,” “assemblage,” “class” or “group” in both Pāḷi and Sanskrit. It is most commonly used in reference to the Buddhist texts of the Sutta Piṭaka but can also refer to the monastic divisions of Theravāda Buddhism.
Pali – Pali (Pāli) is a Prakrit language native to the Indian subcontinent. It is widely studied because it is the language of much of the earliest extant literature of Buddhism as collected in the Pāli Canon or Tipiṭaka and is the sacred language of some religious texts of Hinduism and all texts of Theravāda Buddhism.
Samadhi – Samadhi (Sanskrit: समाधि, Hindi pronunciation: [səˈmaːd̪ʱi]), also called samāpatti, in Hinduism, Buddhism, Jainism, Sikhism and yogic schools refers to a state of meditative consciousness. It is a meditative absorption or trance, attained by the practice of dhyāna. In samādhi the mind becomes still. It is a state of being totally aware of the present moment; a one-pointedness of mind.[web 1]
Samsara – Saṃsāra is a Sanskrit word that means “wandering” or “world”, with the connotation of cyclic, circuitous change. It also refers to the theory of rebirth and “cyclicality of all life, matter, existence”, a fundamental assumption of all Indian religions. Saṃsāra is sometimes referred to with terms or phrases such as transmigration, karmic cycle, reincarnation, and “cycle of aimless drifting, wandering or mundane existence”.
The concept of Saṃsāra has roots in the Vedic literature, but the theory is not discussed there. It appears in developed form, but without mechanistic details, in the early Upanishads. The full exposition of the Saṃsāra doctrine is found in Sramanic religions such as Buddhism and Jainism, as well as the various schools of Hindu philosophy, after about the mid 1st millennium BCE. The Saṃsāra doctrine is tied to the Karma theory of Indian religions and the liberation from Saṃsāra has been at the core of the spiritual quest of Indian traditions, as well as their internal disagreements. The liberation from Saṃsāra is called Moksha, Nirvana, Mukti or Kaivalya
Sutta (Sutra) – Sutra (IAST: sūtra सूत्र) is a Sanskrit word that means “string” or “thread”. In Indian literary traditions, it also refers to an aphorism or a collection of aphorisms in the form of a manual or, more broadly, a condensed manual or text. Sutras are a genre of ancient and medieval Indian texts found in Hinduism, Buddhism, and Jainism.
In Hinduism, sutra denotes a distinct type of literary composition, a compilation of short aphoristic statements. Each sutra is any short rule, like a theorem distilled into few words or syllables, around which teachings of ritual, philosophy, grammar, or any field of knowledge can be woven. The oldest sutras of Hinduism are found in the Brahmana and Aranyaka layers of the Vedas. Every school of Hindu philosophy, Vedic guides for rites of passage, various fields of arts, law, and social ethics developed respective sutras, which helped teach and transmit ideas from one generation to the next.
In Buddhism, sutra or sutta refers mostly to canonical scriptures, many of which are regarded as records of the oral teachings of Gautama Buddha. The Pali form of the word, sutta, is used exclusively to refer to the scriptures of the early Pali Canon, the only texts recognized by Theravada Buddhism as canonical.
Tathagata – Tathāgata (Sanskrit: [t̪əˈt̪ʰɑːɡət̪ə]) is a Pali and Sanskrit word; Gotama Buddha uses it when referring to himself in the Pāli Canon. The term is often thought to mean either “one who has thus gone” (tathā-gata) or “one who has thus come” (tathā-āgata). This is interpreted as signifying that the Tathāgata is beyond all coming and going – beyond all transitory phenomena. There are, however, other interpretations and the precise original meaning of the word is not certain.
The Buddha is quoted on numerous occasions in the Pali Canon as referring to himself as the Tathāgata instead of using the pronouns me, I or myself. This may be meant to emphasize by implication that the teaching is uttered by one who has transcended the human condition, one beyond the otherwise endless cycle of rebirth and death, i.e. beyond dukkha.
Theravada – Theravāda (Pali, literally “school of the elder monks“) is a branch of Buddhism that uses the Buddha’s teaching preserved in the Pāli Canon as its doctrinal core. The Pali canon is the only complete Buddhist canon which survives in a classical Indic Language, Pali, which serves as the sacred language and lingua franca of Theravada Buddhism. Another feature of Theravada is that it tends to be very conservative about matters of doctrine and monastic discipline. As a distinct sect, Theravada Buddhism developed in Sri Lanka and spread to the rest of Southeast Asia.
Theravada also includes a rich diversity of traditions and practices that have developed over its long history of interactions with varying cultures and religious communities. It is the dominant form of religion in Cambodia, Laos, Myanmar, Sri Lanka, and Thailand, and is practiced by minority groups in Bangladesh, China, Nepal, and Vietnam. In addition, the diaspora of all of these groups as well as converts around the world practice Theravāda Buddhism. Contemporary expressions include Buddhist modernism, the Vipassana movement and the Thai Forest Tradition.
Tipitaka – Tripiṭaka, also referred to as Tipiṭaka, is the traditional term for the Buddhist scriptures. The version canonical to Theravada Buddhism is often referred to as Pali Canon in English. Mahayana Buddhism also reveres the Tripitaka as authoritative but, unlike Theravadins, it also reveres various derivative literature and commentaries that were composed much later.
The Tripitakas were composed between about 500 BCE to about the start of the common era, likely written down for the first time in the 1st century BCE. The Dipavamsa states that during the reign of Valagamba of Anuradhapura (29–17 BCE) the monks who had previously remembered the Tipitaka and its commentary orally now wrote them down in books, because of the threat posed by famine, war. The Mahavamsa also refers briefly to the writing down of the canon and the commentaries at this time. Each Buddhist sub-tradition had its own Tripitaka for its monasteries, written by its sangha, each set consisting of 32 books, in three parts or baskets of teachings: (1) the basket of expected discipline from monks (Vinaya Piṭaka), (2) basket of discourse (Sūtra Piṭaka, Nikayas), and (3) basket of special doctrine (Abhidharma Piṭaka). The structure, the code of conduct and moral virtues in the Vinaya basket particularly, have similarities to some of the surviving Dharmasutra texts of Hinduism. Much of the surviving Tripitaka literature is in Pali, some in Sanskrit, as well as other local Asian languages.
Vajrayāna is usually translated as Diamond Vehicle or Thunderbolt Vehicle, referring to the Vajra, a mythical weapon which is also used as a ritual implement.
Vipassana – Vipassanā (Pāli) or vipaśyanā (Sanskrit: विपश्यना; Burmese: ဝိပဿနာ; Sinhalese: විපස්සනා; Chinese: 觀 guān; Standard Tibetan: ལྷག་མཐོང་, lhaktong; Wyl. lhag mthong) in the Buddhist tradition means insight into the true nature of reality, namely as the Three marks of existence: impermanence, suffering or unsatisfactoriness, and the realisation of non-self. Presectarian Buddhism emphasized the practice of Dhyana, but early in the history of Buddhism Vipassanā gained a prominent place in the teachings.
Vipassanā meditation has been reintroduced in the Theravada-tradition by Ledi Sayadaw and Mogok Sayadaw and popularized by Mahasi Sayadaw, V.R. Dhiravamsa,  S. N. Goenka, and the Vipassana movement,  in which mindfulness of breathing and of thoughts, feelings and actions are being used to gain insight into the true nature of reality. Due to the popularity of Vipassanā-meditation, the mindfulness of breathing has gained further popularity in the west as mindfulness.
Please note that these are NOT my definitions but those of Wikipedia. As time goes by I may well update them as my own education proceeds.