'annularity' originates from Latin, specifically the word 'annularis' (from 'annulus'), where 'annulus' meant 'little ring' or 'ring'.

'annulus' (Latin) gave rise to the adjective 'annular' in Late Latin/Medieval Latin ('annularis'), and English formed the noun 'annularity' by adding the noun-forming suffix '-ity' to 'annular'.

Initially it referred to a 'little ring' or the quality of being ring-like; over time it evolved into the abstract noun meaning 'the state or quality of being ring-shaped' and specialized to include the eclipse sense of 'the ring phase.'