- any of a series of paired linear openings to the exterior from the sides of the pharynx in fishes and some amphibians. They contain the gills
- One of several rudimentary invaginations in the embryonic surface, present during development of air-breathing vertebrates and corresponding to the functional gill slits of aquatic species.
- In primitive chordates and cartilaginous fish, such as sharks, one of several narrow external openings connecting with the pharynx through which water passes to the exterior, thereby bathing the gills. Bony fish and all other vertebrates, including humans, have rudimentary gill slits during their embryonic stage.