A subclass of living and extinct diapsid reptiles. Lepidosauria and their immediate ancestors constitute the Lepidosauromorpha, one of the two major clades of the Diapsida. By definition, Lepidosauria includes the last common ancestor of the living squamates (lizards, snakes, and the limb-reduced burrowing amphisbaenians) and the New Zealand tuatara (Sphenodon) and all of its descendants.

This is a narrower definition than that used in older literature, where Lepidosauria was used as a catchall group for all non-archosaurian diapsids. Lepidosaurs differ from archosaurs in that the lower temporal arcade (the inferior border of the lower temporal fenestra) is typically incomplete, there is never an antorbital fenestra in front of the orbit or a mandibular fenestra in the lower jaw, and the teeth are generally fused in position (acrodont or pleurodont) rather than implanted in sockets (thecodont). Other lepidosaurian characters include a specialized skin-shedding mechanism, paired male hemipenes (copulatory organs that characterize lizards and snakes), fracture planes in the caudal vertebrae that allow the tail to be shed if grabbed by a predator, and specialized knee, foot, and ankle joints that improve locomotion. As in mammals, the ends of lepidosaurian long bones develop separate centers of ossification (epiphyses) that fuse to the shaft at the end of skeletal growth.