'postreaction' originates from the prefix 'post-' meaning 'after' and the word 'reaction' from Latin 'reactio', where 're-' meant 'again' and 'actio' meant 'action'.

'reactio' transformed into the English word 'reaction', and with the addition of the prefix 'post-', it became 'postreaction'.

Initially, it meant 'an action occurring after another', and this meaning has largely remained the same in modern usage.