'synapsid' originates from New Latin 'Synapsida', ultimately from Greek 'syn-' meaning 'together' and 'apsis' meaning 'arch' or 'joining'.

'synapsid' changed from New Latin 'Synapsida', a scientific-taxonomic formation based on Greek roots, and entered modern English via scientific Latin and palaeontological literature.

Initially it referred specifically to the taxonomic group defined by skull anatomy (a single temporal opening); over time it has come to be used more broadly for members of the mammal lineage, including both extinct relatives and living mammals.