In mountaineering, a bivouac, bivy, bivy sack, or bivi bag is an extremely lightweight alternative to traditional tent systems. Very popular among climbers and minimalist campers, a bivy sack at its barest is a thin waterproof fabric shell designed to slip over a sleeping bag, providing an additional 5 to 10 °F of insulation and forming an effective barrier against wind and rain. A drawback of a simple bivy sack is the humidity that condenses at the inner side leaving the occupant or the sleeping bag clammy. Better bivy sacks consist of Gore-Tex (or a similar breathable fabric) to allow the humidity to pass.
Nowadays there also exists the bivouac shelter or "bivy shelter", a compromise between bivy sack and single-person tent. Using hoops or poles, a bivy shelter is usually supported along its length just enough to keep the fabric off of the occupant, and especially to provide some additional breathing room around the head.
In contrast to a bivy shelter, a traditional bivy sack typically cinches all the way down to the user's face, leaving only a small hole to breathe or look through. Many campers gladly accept the increased weight of a bivy shelter for the huge increase in comfort it affords. However, the traditional bivy sack certainly still holds its place among climbers and backpackers, and is frequently carried on long or dangerous hikes and climbs as a compact emergency shelter.
In the UK, Bivy shelters have become very popular amongst Carp fisherman, who, in pursuit of their quarry, fish throughout the night. This upsurge in popularity has increased competition amongst manufacturers and designs have become more advanced. In the UK, they are commonly called 'bivvies'.