Wiktionary
n. waterproof fabric shell designed to slip over a sleeping bag
Wikipedia
A bivouac sack (also known as a 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 bivouac sack has a larger, similar counterpart, called a bivouac shelter.
The main benefit of a bivouac sack shelter is speed of setup and ability to use in a tiny space as compared to tent-like shelters. A bivouac sack is a good choice for hikers or climbers who have to camp in tight areas or in unknown areas. It is far better on average to be in a tent-like shelter in bad weather than in a bivy sack. A bivy sack coated with water from rain is not very breathable. For good weather a bivy sack can be a good choice with star viewing comfort, in bad weather not so much. Check the weather forecast and if good then bivy sack, if not then tent, or bivy sack with tarp.