1. (in zoology) A respiratory organ used by aquatic animals to obtain oxygen from the surrounding water. A gill consists essentially of a membrane or outgrowth from the body, with a large surface area and a plentiful blood supply, through which diffusion of oxygen and carbon dioxide between the water and blood occurs. Fishes have internal gills, formed as outgrowths from the pharynx wall and contained within gill slits. Water entering the mouth is pumped out through these slits and over the gills. The gills of most aquatic invertebrates and amphibian larvae are external gills, which project from the body so that water passes over them as the animal moves. Most aquatic vertebrates and invertebrates with gills also use them for excreting nitrogenous wastes and maintaining the ionic composition of body fluids. See chloride secretory cell.

2. (in botany) One of the ridges of tissue that radiate from the centre of the underside of the cap of mushrooms. The spores are produced on these gills.