'synonym' originates from Late Latin, specifically the word 'synonymum', ultimately from Greek 'synōnymon' (συνώνυμον), where 'syn-' meant 'together' and 'onoma' (or 'onyma') meant 'name'.

'synonym' changed from Greek 'synōnymon' into Late Latin 'synonymum' and entered English (via Medieval/early modern channels) as 'synonim'/'synonym', becoming the modern English form 'synonym'.

Initially it meant 'the same name', but over time it evolved to mean 'a word having the same or a very similar meaning as another word'.