'simultaneity' originates from Latin, specifically the word 'simultaneus,' where 'simul' meant 'together' and 'taneus' was a suffix indicating 'pertaining to.'

'simultaneus' transformed into the French word 'simultanéité,' and eventually became the modern English word 'simultaneity' through Middle English.

Initially, it meant 'occurring at the same time,' and this meaning has largely remained the same in modern usage.