'Synapsida' originates from Neo-Latin, formed from Greek elements 'syn-' meaning 'together' and 'apsis' (or a related Greek root) historically referring to an arch or opening, combined to create a taxonomic name.

'Synapsida' was coined in scientific Latin usage to name the group of amniotes with a single temporal opening; the formation follows the standard Neo-Latin practice of creating clade names from Greek roots and has been used in zoological and paleontological literature since the 19th–20th centuries.

Initially constructed from Greek elements referring to structure (an arch/opening), the term has come to denote specifically the clade of amniotes characterized by a single temporal fenestra, including mammals.