'diapsid' originates from Modern Latin (New Latin), specifically the word 'Diapsida', where the Greek prefix 'di-' meant 'two' and the Greek root 'apsis' meant 'arch' or 'opening'.

'diapsid' changed from the New Latin taxonomic name 'Diapsida', which was formed from Greek elements; this term was adopted into English scientific vocabulary as 'diapsid' (singular) and 'diapsids' (plural).

Initially, it meant 'having two temporal openings (fenestrae) in the skull'; over time this anatomical descriptor became the name of a major reptile clade and is still used in that sense.