A survival bag or bivi bag (short for bivouac) is a person-sized waterproof bag, canonically orange in colour, designed for use in wilderness situations to avert the threat of hypothermia from exposure. It is reasonably light, made from strong, tear-proof plastic, and provides some amount of thermal insulation.
Survival bags are recommended (or even compulsory) for carrying in hill walking and mountain marathonning. They can be used as an emergency tent substitute if a day trip is accidentally prolonged, or (preferably in combination with a sleeping bag) to warm up a person suffering hypothermia, from exposure, or whose clothes have been soaked (e.g. from a river crossing); for that purpose the person should be naked to ensure energy is not wasted heating up wet clothes. They are also sometimes used in place of groundsheets, or for sliding down hills.