'apsid' originates from Greek via Latin: from Greek 'ἀψις' (apsis) meaning 'arch' or 'vault', through Latin 'apsis' and Late Latin forms.

'apsid' changed from Latin/Medieval Latin 'apsis' (and Old French influence in related architectural terms) and eventually appeared in modern English as 'apsid', used as a variant of 'apse'/'apsis'.

Initially, it meant 'arch' or 'vault' in the architectural sense; over time the form has also been used as a variant of 'apsis' for orbital extreme points, while the architectural sense remains primary in many contexts.