'aplanacy' originates from Greek elements, specifically 'a-' (a privative prefix) and 'planos' (wandering), where 'a-' meant 'not' and 'planos' meant 'to wander or stray'.

'aplanacy' came into technical English via New Latin/French formations related to 'aplanatic' (French aplanatique, New Latin aplanaticus); the adjective 'aplanatic' entered English and the noun 'aplanacy' was later formed with the suffix '-acy'.

Initially formed from elements meaning 'not wandering' (literally 'not straying'), it developed a specialized technical sense in optics as 'free from image wandering or deviation' and now means 'free from optical aberration'.