Synapsids ('fused arch') are a group of animals that includes mammals and every animal more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having a temporal fenestra, an opening low in the skull roof behind each eye, leaving a bony arch beneath each; this accounts for their name. Primitive synapsids are usually called pelycosaurs; more advanced mammal-like ones, therapsids. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem mammals" (or sometimes "protomammals") under cladistic terminology. Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes, the other being the sauropsids (reptiles and birds). Their distinctive temporal fenestra developed in the ancestral synapsid about 324 million years ago (mya) during the late Carboniferous period.

Synapsids were the largest terrestrial vertebrates in the Permian period, 299 to 251 million years ago. As with almost all groups then extant, their numbers and variety were severely reduced by the Permian-Triassic extinction. Though some species survived into the Triassic period, archosaurs became the largest and most numerous land vertebrates in the course of this period. Few of the nonmammalian synapsids outlasted the Triassic, although survivors persisted into the Cretaceous. However, as a phylogenetic unit, they included the mammals as descendants, and in this sense synapsids are still very much a living group of vertebrates. After the Cretaceous–Paleogene extinction event, the synapsids (in the form of mammals) again became the largest land animals.