'diapsida' originates from New Latin (taxonomic use), ultimately from Greek elements: 'di-' meaning 'two' and 'apsis' meaning 'arch' or 'vault' (used for an opening).

'di-' + 'apsis' (Greek) was Latinized in New Latin as 'Diapsida' for a taxonomic group; the term was adopted into modern English scientific usage to name the clade.

Initially a literal formation meaning 'those with two arches/openings' (referring to skull openings); it evolved into the formal name for the reptile clade characterized by two temporal fenestrae.