'reactivity' originates from the Latin word 'reactivus', where 're-' meant 'again' and 'activus' meant 'active'.

'reactivus' transformed into the French word 'réactif', and eventually became the modern English word 'reactive' through Middle English.

Initially, it meant 'to act again or respond', but over time it evolved into its current meaning of 'the degree to which something is reactive'.