Zazen is the seated meditation practice at the heart of Zen Buddhism, which developed in China as Chan Buddhism before spreading to Japan. In zazen, practitioners sit in a stable posture — typically full or half lotus — with eyes half-open and cast downward, and simply sit. Rather than concentrating on an object or visualizing anything, zazen is the practice of just sitting — alert, upright, and present. Some Zen traditions use koans — paradoxical questions like 'What is the sound of one hand clapping?' — as objects of contemplation. Zazen is typically practiced in a zendo (meditation hall) under the guidance of a teacher.

Reference:

Wikipedia: Zazen

https://en.wikipedia.org/wiki/Zazen