'isotonicity' originates from New Latin/modern formation using Greek elements 'iso-' and 'tonos', where 'iso-' meant 'equal' and 'tonos' meant 'tension' or 'stretch'.

'isotonic' was coined in the 19th century in physiology/chemistry from Greek elements; the noun 'isotonicity' was formed later by adding the suffix '-ity' (from Latin '-itas' via Old French) to create an abstract noun meaning the state or quality of being isotonic.

Initially related to the idea of equal tension or tone (e.g., muscular tone), the term's primary usage shifted to denote equality of osmotic pressure between solutions, which is the dominant modern meaning.