Zazen (坐禅; Chinese: zuò chán pinyin or tso-chan Wade-Giles) is at the heart of Zen Buddhist practice. The aim of zazen is just sitting, "opening the hand of thought". This is done either through koans, Rinzai's primary method, or whole-hearted sitting (shikantaza), the Soto sect's method. (Rinzai and Soto are the main extant Zen schools in Japan; they both originated in China as the Linji and Caodong schools, respectively.) Once the mind is able to be unhindered by its many layers, one will then be able to realize one's true Buddha nature. In Zen Buddhism, zazen (literally "seated meditation") is a meditative discipline practitioners perform to calm the body and the mind and experience insight into the nature of existence and thereby gain enlightenment (satori).
The posture of zazen is seated, with folded legs and hands, and an erect but settled spine. The legs are folded in one of the standard sitting styles (see below). The hands are folded together into a simple mudra over the belly. In many practices, one breathes from the hara (the center of gravity in the belly) and the eyelids are half-lowered, the eyes being neither fully open nor shut so that the practitioner is not distracted by outside objects but at the same time is kept awake.
Long periods of zazen, usually performed in groups at a zendo (meditation hall), may alternate with periods of kinhin (walking meditation). The beginning of a zazen period is traditionally announced by ringing a bell three times (shijosho), and the end of a round by ringing the bell once (hozensho). Before and after sitting, Zen practitioners perform a gassho bow to their seat, to fellow practitioners and to the teacher.
In addition, it is not uncommon for modern practitioners to sit zazen in a chair, often with a wedge behind the lower back to help maintain the natural curve of the spine.
Very generally speaking, zazen practice is taught in one of three ways.
The initial stages of training in zazen will usually emphasize concentration. By focusing on the breath at the hara, often aided by counting, one builds up the power of concentration, or joriki. At some Zen centers, the practice of mentally repeating a mantra with the breath is used in place of counting breaths for beginners. In some communities, or sanghas, the practice is continued in this way until there is some initial experience of samadhi or "one-pointedness" of mind. At this point the practitioner moves to one of the other two methods of zazen.
Having developed the power of concentration, the practitioner can now focus his or her attention on a koan as an object of meditation. Since koans are not solvable by the intellectual reasoning, koan introspection is designed to shortcut the intellectual process leading to direct realization.
Shikantaza (just sitting)
Shikantaza is objectless meditation, in which the practitioner does not use any specific object of meditation, but uses the power developed in concentration to remain completely aware of all phenomena that arise and pass in the present moment.
Concentration practice in Zen is likened to the practice of samatha (concentration) in other schools of Buddhism. One apparent difference is that the eyes remain open in zazen, whereas in the Theravada tradition they do not. Tibetan Buddhist practitioners keep their eyes open during samatha practice.
Concentration is foundational to most other forms of meditation in Buddhism. In actuality, all meditative practices, Buddhist and non-Buddhist, take concentration to execute, and therefore are concentration practices in and of themselves. Some teachers do not teach concentration as a separate practice, believing that it is developed through other practices.
Koan introspection and shikantaza are more likened to the vipashyana (insight) practice in Theravada, but are sometimes considered to be a condensation of vipashyana and samatha into a single practice. For this reason, shikantaza can also be referred to as samatha-vipashyana. Similarly, koan introspection, while leading to insight, requires an immense amount of concentration on the object of meditation (the koan).