1 any of the very small holes in the surface of a leaf or the STEM (= the main long thin part) of a plant that allow gases to pass in and out （植物的）气孔

2 a small opening like a mouth in some simple creatures （某些低级动物的）嘴状小孔

3 (in medicine) a hole that is made from the surface of the body to one of the tubes inside the body （医学）（手术过程中在人体切开的）人造口