Definition of synapsid
: any of a subclass (Synapsida) of extinct chiefly Permian and Triassic reptiles (such as the pelycosaurs) that resembled mammals, had a single pair of lateral temporal skull openings, and are usually held to be ancestral to mammals
Seen and Heard
What made you want to look up synapsid? Please tell us where you read or heard it (including the quote, if possible).