A hydration system is an apparatus used in recreation and other sustained outdoor activities. It is intended to help its user carry liquid to support the physical effort involved in the activity. Such systems for consumers were first sold to cyclists, and by the 1990s had also found a substantial market among hikers. Familiar commercial models can also be recognized occasionally worn by western military personnel in southwest Asia.
In practice, such a system is almost always a commercially manufactured unit that features at least
a flexible bladder of one or a few liters' (quarts') capacity with some means, usually a screwtop, to fill and then reliably seal it,
a light hose to convey the beverage to the user's mouth, and
a bite valve that starts and stops the flow through the hose with minimal effort.