'isotonic' originates from Greek via New Latin, specifically from Greek 'isotonikos' (ἰσοτονικός), where 'isos' meant 'equal' and 'tonos' meant 'tension' or 'stretch'.

'isotonic' changed from the Greek word 'isotonikos' into New Latin 'isotonicus' and later entered modern English as 'isotonic' (19th century usage in physiology and chemistry).

Initially it meant 'having equal tension' (in the literal sense of equal tone/tension); over time it came to be used for 'having equal osmotic pressure' and for describing a type of muscle contraction or beverage with that property.