'equilateral' originates from Latin, specifically the word 'aequilateralis,' where 'aequi-' meant 'equal' and 'lateralis' meant 'of sides.'

'aequilateralis' changed from Medieval Latin to Old French as 'equilateral,' and eventually became the modern English word 'equilateral.'

Initially, it meant 'having equal sides,' and this meaning has remained the same in modern usage, especially in geometry.