a small round cushion that zen buddhists (and others) sit on to meditate

A small round cushion used as a seat in zazen.

(Japanese) The cushion used for zazen or sitting meditation.

pillow on which one sits

(Japanese): a round meditation cushion.

A zafu (座蒲 in Japanese or 蒲团 in Chinese) is a round cushion, about 35 cm (14 inches) in diameter, and often about 20 cm (8 inches) high, when fluffed. Although in U.S. English, zafu is often translated as "sewn seat", that is not the meaning of the Japanese kanji.