1. A small paired opening that occurs on each side of the head in cartilaginous fish. It is the reduced first gill slit, its small size resulting from adaptations of the skeleton for the firm attachment of the jaws. In modern teleosts (bony fish) the spiracle is closed up. In tetrapods the first gill slit develops into the middle ear cavity.

2. Any of the external openings of the tracheae along the side of the body of an insect.