Isotonic has many definitions in health and science.
Chemistry & Biochemistry
An isotonic solution is one that has an equal amount of dissolved solute in it compared to the things around it. Sports drinks are said to be isotonic if they contain concentrations of salt levels that mimic the osmolality of blood.
Fitness & Training
Isotonic literally means equal tension. An isotonic contraction is a contraction in which the tension remains constant as the muscle shortens or lengthens.