'parallelity' ultimately derives from Greek 'parallēlos' (παράλληλος), where 'para-' meant 'beside' and 'allēl' (from 'allēloi') meant 'one another'; the noun-forming suffix '-ity' comes from Latin '-itas' via Old French '-ité'.

Greek 'parallēlos' passed into Latin and then into Old French and Middle English as forms like 'parallel' or 'paralel'; the abstract noun formed with the Latinate suffix ('-ity' from '-itas') produced modern English 'parallelity'.

Originally conveying the idea 'lying beside one another', the sense evolved to the abstract notion 'the state or quality of being parallel' used in modern English.