'diapsid' originates from New Latin/Greek combining forms: Greek 'di-' (from Greek 'dis') meaning 'two' and Greek 'apsis' meaning 'arch' or 'vault' (used here to refer to an opening), combined and Latinized in scientific nomenclature.

'diapsid' was formed in scientific (zoological/paleontological) usage in the late 19th century as the taxonomic name 'Diapsida'; English later adopted the adjectival/noun form 'diapsid' to refer to members of that group.

Initially coined to denote 'having two (arch/opening) features' in the skull, its meaning has remained focused on reptiles with two temporal fenestrae and the taxonomic group defined by that trait.