A diapsid is a reptile with two fenestrae (holes) behind the eye on each side of its skull. Most reptiles are diapsids, and the condition does not occur in mammals. The function of the holes is to improve jaw movements, and to reduce the weight of the skull. The holes give places for muscle attachment; also, as jaw muscles contract, the holes allow space for the muscles to bulge.
Some aquatic reptile groups lost the lower of the two openings, leaving just one behind the eye. This is called the euryapsid condition. It is fairly certain that this is secondary, and that the groups were originally diapsids. The groups include: