You'll find many more key definitions at the Lion's Roar Buddhist Glossary.

Avalokiteshvara (Sanskrit) — The bodhisattva of compassion. Also widely known by names such as Chenrezig in Tibet, Kanzeon/Kannon in Japan, Kuan Yin in Chinese Buddhism, and others. 

Related reading: Who Is Avalokiteshvara?

Bodhicaryavatara (Sanskrit) — The title of Shantideva’s famed text, translated in English as A Guide to the Bodhisattva's Way of Life, The Way of the Bodhisattva, and variations thereof.

bodhicitta (Sanskrit) — “Enlightenment mind”; the state of mind of the bodhisattva, striving toward enlightenment and infused with the compassionate motivation to help others. 

Related reading: A Bodhicitta Practice 

bodhisattva (Sanskrit) — Literally, “enlightenment being.” In Mahayana Buddhism, one who practices with the vow and motivation to put others before oneself, which may include forgoing enlightenment until all others have achieved it. In other Buddhist schools, the term is often used to refer specifically to the historical Buddha, Shakyamuni, before his enlightenment. 

Recommended reading: How to Be a Bodhisattva

dharma (Sanskrit) — The teachings of Buddhism. Can also refer to non-Buddhist teachings and insights.

dukkha (Pali) — “Suffering” is the most common translation of the Sanskrit word dukkha, which is also translated as dissatisfaction. Dukkha describes the sense that experience is suffused with unsatisfactoriness — ranging from subtle states like longing and ennui to extreme states like grief and agony.

Related reading: What Is Suffering?

Dzogchen (Tibetan) — The practice of "Great Perfection" or "Great Completion.” As Yongey Mingyur Rinpoche wrote in Lion’s Roar magazine, “Dzogchen is treasured above all other practices in the Nyingma school of Vajrayana Buddhism because it helps us connect directly with our own enlightened nature.”

guru — In Vajrayana Buddhism, a guru is a teacher whom students regard as enlightened. This is an advanced form of practice, and it is recommended that students exercise caution before accepting a guru. In principle, gurus are dedicated to helping others and adept at helping their students realize their true nature.

karma (Sanskrit) — The law and workings of cause and effect. The law of karma says that all things are interconnected, all actions have consequences, and all consequences are the result of past actions. Buddhism also teaches that, while karma is very complex, positive actions generally reap positive consequences and negative actions generally reap negative consequences.

Related reading: What Is Karma?

kleshas (Sanskrit) — Often referred to as “defilements.” As Pema Chödrön has put it in Lion’s Roar magazine, klesha “refers to a strong emotion that reliably leads to suffering. It’s sometimes translated as neurosis. In essence, kleshas are dynamic, ineffable energy, yet it’s energy that easily enslaves us and causes us to act and speak in unintelligent ways.” May also be referred to as kilesas (Pali). 

Related reading: Pema Chödrön on Waking Up — and Benefiting Others

karuna (Sanskrit/Pali) — Compassion.

Related reading: Only Genuine Compassion Will Do

Mahamudra (Sanskrit) — A form of meditation taught in the Vajrayana, it begins with shamatha or calm abiding, and in time helps the practitioner develop clarity and insight into emptiness, or shunyata.

Mahayana (Sanskrit) — A later development in Buddhism that typically emphasizes the ideal of the bodhisattva. Pure Land and Zen are both examples of Mahayana school.

mantra (Sanskrit) — A series of syllables (often, but not always, Sanskrit) meant to be recalled/recited as part of contemplative practice. 

Related reading: Modern Mantras

Mara(s) (Sanskrit) — When singular: mythical demon who tempted Siddhartha Gautama, the historical Buddha to stray from his meditative aspirations; when plural (maras): may refer to multiple kinds of temptors or temptations. 

prajñā (Sanskrit), or paññā (Pāli) — Wisdom.

rigpa (Tibetan) — As relates to Dzogchen teachings, knowing the nature of the mind.

samadhi (Sanskrit) — meditative absorption.

samsara (Sanskrit) — The ongoing cycle of life—birth and death and rebirth. Due to our ignorance, we go through this cycle with a sense of suffering and dissatisfaction. Buddhist practice is, to put it very simply, about undoing our ignorance and transcending our traditional relationship to samsara.

Related reading: Why Does Buddhism Talk About Suffering So Much?

sangha (Sanskrit) — sangha is a community that practices the dharma together. It’s one of the Three Jewels in which Buddhists take refuge, along with the buddha and the dharma.

Related reading: What Is Sangha?

shamatha (Pali: samatha, Sanskrit: shamatha) — tranquility; refers to the mental peace and stability developed in meditation. Insight and tranquility are the two qualities of mind that are developed in meditative practice.

Shantideva (person) — Eighth-century Indian adept and author of A Guide to the Bodhisattva’s Way of Life (Bodhicaryavatara).

shunyata (Pali) — Emptiness. Emptiness is the central insight of Buddhism, and what makes it unique among religions. According to Buddhism, neither we, nor other beings, nor any phenomenon in the universe, has a permanent, separate, and independent core, soul, or identity.

Related reading: Understanding Emptiness and Interdependence

Siddhartha Gautama (Person) — Siddhartha Gautama, also known as Shakyamuni Buddha or simply the Buddha, meaning the Awakened One, was a spiritual leader from the Indian subcontinent who lived roughly 2,600 years ago. He is particularly known for teachings on the Middle Way and the Four Noble Truths. Buddhism was founded on his teachings.

skandhas (Sanskrit) — The five skandhas are the constituent parts that make up living beings. Skandha means “heap.” They are referred to as heaps because they are collections of parts without any central core. The five skandhas are: form, feeling, perception, formation, mental formation, and consciousness.

sutra/sutras (Sanskrit) / sutta/suttas (Pali) — Discourses of the Buddha; that is, oral teachings attributed to him.

Related reading: What Is a Sutra?

Tonglen (Tibetan) — Tonglen literally means “giving and taking.” It is a meditative practice of visualizing oneself accepting the suffering of another, transforming that suffering into happiness, and returning that happiness to the other. Tonglen is a technique for developing bodhichitta.

Related reading: Helping Others, and Yourself, Through Tonglen

Vajrayana (Sanskrit) — Literally, “diamond vehicle.” A later-developing tradition of Buddhism, most famously associated with Buddhism in Tibet and the Himalayas, that stresses esoteric teachings. Considered a further form of Mahayana, the Himalayan Vajrayana tradition is composed of the Nyingma, Kagyu, Gelug, and Sakya schools.

Recommended reading: Vajrayana Explained

See many more key definitions at the Lion's Roar Buddhist Glossary.