Synapsids are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptiles and birds. The group includes mammals and every animal more closely related to mammals than to sauropsids. Unlike other amniotes, synapsids have a single temporal fenestra, an opening low in the skull roof behind each eye orbit, leaving a bony arch beneath each; this accounts for their name. The distinctive temporal fenestra developed about 318 million years ago during the Late Carboniferous period, when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.

Traditionally, non-mammalian synapsids were believed to have evolved from reptiles, and therefore described as mammal-like reptiles in classical systematics, and primitive synapsids were also referred to as pelycosaurs, or pelycosaur-grade synapsids. These paraphyletic terms have now fallen into disfavor and are only used informally (if at all) in modern literature, because it is now known that synapsids are not reptiles, nor are they part of reptilian lineage in a cladistical sense. They are now more correctly referred to as stem mammals, and sometimes as proto-mammals, or paramammals.

Synapsids were the largest terrestrial vertebrates in the Permian period, 299 to 251 million years ago, equalled only by some large pareiasaurs at the end of the Permian. Most lineages of pelycosaur-grade synapsids were replaced at the end of Early Permian by the more advanced therapsids. Synapsid numbers and variety were severely reduced by the Permian–Triassic extinction event. Only the therapsid dicynodonts and eutheriodonts (consisting of Therocephalia and Cynodontia) are known to have continued into the Triassic period. The cynodont group Probainognathia, which includes Mammaliaformes (mammals and their closer ancestors), were the only synapsids to survive beyond the Triassic.

During the Triassic, the sauropsid archosaurs became the largest and most numerous land vertebrates, and gave rise to the dinosaurs. When all non-avian dinosaurs were wiped out by the Cretaceous–Paleogene extinction event, the mammalian synapsids diversified again to become the largest land and marine animals on Earth.