Zen is a school of Mahāyāna Buddhism. The word Zen is from the Japanese pronunciation of the Chinese word Chán (禪), which in turn is derived from the Sanskrit word dhyāna, which can be approximately translated as “meditation” or “meditative state”.
Zen emphasizes experiential Wisdom in the attainment of enlightenment. As such, it de-emphasizes theoretical knowledge in favor of direct realization through meditation and dharma practice. The teachings of Zen include various sources of Mahāyāna thought, including the Prajñāpāramitā literature and the teachings of the Yogācāra and Tathāgatagarbha schools.
The emergence of Zen as a distinct school of Buddhism was first documented in China in the 7th century CE. From China, Zen spread south to Vietnam, and east to Korea and Japan. As a matter of tradition, the establishment of Zen is credited to the South Indian Pallava prince-turned-monk Bodhidharma, who came to China to teach a “special transmission outside scriptures, not founded on words or letters”.
Definition courtesy Wikipedia.