Zen (Japanese pronunciation: [dzeꜜɴ, dzeɴ]; from Chinese: Chán; in Korean: Sŏn, and Vietnamese: Thiền) is a Mahayana Buddhist tradition that developed in China during the Tang dynasty by blending Indian Mahayana Buddhism, particularly Yogacara and Madhyamaka philosophies, with Chinese Taoist thought, especially Neo-Daoist. Zen originated as the Chan school (禪宗, chánzōng, 'meditation school') or the Buddha-mind school (佛心宗, fóxīnzōng), and later developed into various sub-schools and branches.

Chan is traditionally believed to have been brought to China by the semi-legendary figure Bodhidharma, an Indian (or Central Asian) monk who is said to have introduced dhyana teachings to China. From China, Chán spread south to Vietnam and became Vietnamese Thiền, northeast to Korea to become Seon Buddhism, and east to Japan, becoming Japanese Zen.

Zen emphasizes meditation practice, direct insight into one's own Buddha nature (見性, Ch. jiànxìng, Jp. kenshō), and the personal expression of this insight in daily life for the benefit of others. Some Zen sources de-emphasize doctrinal study and traditional practices, favoring direct understanding through zazen and interaction with a master (Jp: rōshi, Ch: shīfu) who may be depicted as an iconoclastic and unconventional figure. In spite of this, most Zen schools also promote traditional Buddhist practices like chanting, precepts, walking meditation, rituals, monasticism and scriptural study.

With an emphasis on Buddha-nature thought, intrinsic enlightenment and sudden awakening, Zen teaching draws from numerous Buddhist sources, including Sarvāstivāda meditation, the Mahayana teachings on the bodhisattva, Yogachara and Tathāgatagarbha texts (like the Laṅkāvatāra), and the Huayan school. The Prajñāpāramitā literature, as well as Madhyamaka thought, have also been influential in the shaping of the apophatic and sometimes iconoclastic nature of Zen rhetoric.

The word Zen is derived from the Japanese pronunciation (kana: ぜん) of the Middle Chinese word 禪 (Middle Chinese: [dʑian]; pinyin: Chán), which in turn is derived from the Sanskrit word dhyāna (ध्यान), which can be approximately translated as 'contemplation', 'absorption', or 'meditative state'.

The actual Chinese term for the "Zen school" is 禪宗 (pinyin: Chánzōng), while "Chan" just refers to the practice of meditation itself (Chinese: 習禪; pinyin: xíchán) or the study of meditation (Chinese: 禪學; pinyin: chánxué) though it is often used as an abbreviated form of Chánzong.

Zen is also called 佛心宗, fóxīnzōng (Chinese) or busshin-shū (Japanese), the "Buddha-mind school", from fó-xīn, 'Buddha-mind'; "this term can refer either to the (or a) Buddha's compassionate and enlightened mind, or to the originally clear and pure mind inherent in all beings to which they must awaken." Busshin may also refer to Buddhakaya, the Buddha-body, "an embodiment of awakened activity".

"Zen" is traditionally a proper noun as it usually describes a particular Buddhist sect. In more recent times, the lowercase "zen" is used when discussing a worldview or attitude that is "peaceful and calm". It was officially added to the Merriam-Webster dictionary in 2018.