'atomity' originates from Modern English formation combining 'atom' and the noun-forming suffix '-ity', where 'atom' meant 'indivisible' and '-ity' denotes a state or condition.

'atom' changed from Greek 'atomos' (ἀτόμος, a- 'not' + tomos 'cut') into Latin and then Middle English 'atom', and in Modern English the suffix '-ity' was attached to form 'atomity'.

Initially it meant 'the state of being indivisible', and this core meaning has largely remained, though the term is now rare and often replaced by 'atomicity'.