Sitting meditation as practiced in the Zen School of Buddhism. In Zen Buddhism, zazen (literally "seated meditation") is a meditative discipline that is typically the primary religious practice. The precise meaning and method of zazen varies from school to school, but in general it can be regarded as a means of insight into the nature of existence. In the Japanese Rinzai school, zazen is usually associated with the study of koans. The Sōtō School of Japan, on the other hand, only rarely incorporates koans into zazen, preferring an approach where the mind has no object at all, known as shikantaza.
Example via www.ramdass.org: Meditation and Expectations
A branch of Mahayana originating in China that originally emphasizes non-dualism and intuition. Modern monastic forms have a strong emphasis on zazen (Korean) or on zazen combined with militaristic top-down hazing (Japanese).
Zen is a school of Mahayana Buddhism that developed in China during the Tang dynasty as Chán. From China, Chán spread south to Vietnam, northeast to Korea and east to Japan, where it became known as Japanese Zen.
Zen emphasizes rigorous meditation-practice, insight into Buddha-nature, and the personal expression of this insight in daily life, especially for the benefit of others. As such, it deemphasizes mere knowledge of sutras and doctrine and favors direct understanding through zazen and interaction with an accomplished teacher.
The teachings of Zen include various sources of Mahāyāna thought, especially Yogācāra, the Tathāgatagarbha Sutras and Huayan, with their emphasis on Buddha-nature, totality, and the Bodhisattva-ideal. The Prajñāpāramitā literature and, to a lesser extent, Madhyamaka have also been influential in the shaping of the "paradoxical language" of the Zen-tradition.
In Zen, a hall where zazen is practiced. Zen-dō or senbutsu-jō is a Japanese "meditation hall". In Zen Buddhism, the zen-dō is a spiritual dōjō where zazen (sitting meditation) is practiced. A full-sized Zen Buddhist temple will typically have at least one zen-dō as well as a hon-dō ("main hall", but sometimes translated as "Buddha hall"), which is used for ceremonial purposes, plus a variety of other buildings with different functions. However, any place where people go to practice Zen can be referred to as a zen-dō.