A bivouac is a temporary military encampment that is usually formed in an unsheltered area. The early meaning of the term was for an army to camp for the night, usually without a tent or cover. Later it also came to refer to a simple tent or shelter, known as a 'hootchie' made from one or two sheets of waterproof fabric and some strong hootchie cord.

There are many different ways to put up a bivouac shelter. The most common method is use one bivouac sheet as the roof of the shelter and a second as the groundsheet. The 'roof' flysheet is suspended along in its ridge line by a cord tied between two trees which are a suitable distance apart. The four corners of the flysheet are then either pegged out or tied down to other trees. Care must be taken to leave a gap between the ground and the sheet to ensure that you can see out and that there is enough air flow to stop condensation.

When making camp in a tropical situation, where sleeping off the ground is desirable, it is a common practice to string up a hammock underneath the bivouac rather than use a groundsheet.

In mountaineering terms, a bivouac, bivy, or bivy sack, 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 5-10 degrees of insulation and forming an effective barrier against wind and rain. Nowadays there also exists the 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 (a traditionally 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.

The word bivouac is also used for a structure formed by migratory Army ant and Driver ant colonies, where a nest is constructed out of the living ant worker's own bodies to protect the queen and larvae, and is later deconstructed as the ants move on.

Bivouac is also the name of an album by the punk band Jawbreaker.