http://en.wikipedia.org/wiki/Isotonic
An isotonic solution has an equal amount of dissolved solute in it compared to the things around it. Typically in humans and most other mammals, the isotonic solution is 0.9 weight percent (9 g/L) salt in aqueous solution, this is also known as saline, which is generally administered via an intra-venous drip. It is a medium between hypotonic and hypertonic solutions. Red blood cells normally exist in a 0.9 percent salt solution (saline) with the same concentration of salt in the outside solution.
Athletics
This term is also used in relation to a sports drink with balanced mineralised levels mimicking the osmolality of blood. This property is claimed by an advertising campaign to increase physical endurance. A scientific study, of 6 rowers, attributes a 10% increase in endurance to the extra carbohydrate provided by the isotonic drink.
http://en.wikipedia.org/wiki/Sports_drink
Sports drinks are not to be confused with energy drinks (e.g. Lucozade or Mountain Dew AMP or Monster Energy). Sports drinks are intended to replenish electrolytes, sugar, water, and other nutrients, and are usually isotonic (containing the same proportions as found in the human body). Energy drinks, on the other hand, simply provide lots of sugar and caffeine.