A bivi bag (also known as a bivouac sack, bivy, bivvy, or bivi) is an extremely small, lightweight, waterproof shelter, and an alternative to traditional tent systems. It is used by climbers, mountaineers, hikers, ultralight backpackers, soldiers and minimalist campers. The bivi bag has a larger, similar counterpart, called a bivouac shelter.
Nowadays there also exists the bivouac shelter or "bivi shelter", a compromise between a bivi bag and a single-person tent. Often employing hoops over the head and feet, a bivi shelter is held sufficiently taut to keep the fabric off the occupant inside in order to prevent condensation from soaking into bedding. This style of shelter also provides some additional breathing room around the head. Its design also enables the occupant to roll over comfortably, thereby allowing comfortable sleeping in any desired position without altering the orientation of the top surface of the bivi shelter.
Many campers gladly accept the increased carrying weight of a bivi shelter for the perceived increase in comfort it affords. However, the traditional bivi bag still holds its place among mountain climbers and backpackers, and is frequently carried on long or dangerous hiking or hill walking expeditions and high mountain climbs as a compact emergency shelter.