Bivvy sack, also known as a bivy bag or bivi, is a protective shelter used by mountaineers, trekkers, and adventurers alike to spend a night outdoors. Although the term bivvy sack is the most common, there are several other synonyms for it. Some of the commonly used synonyms are ultralight shelter, minimalist shelter, survival shelter, emergency shelter, and sleeping bag cover. The use of these synonyms varies with individual preferences and the level of protection needed in the outdoor environment. Regardless of the synonym used for bivvy sack, it remains an essential gear for any outdoor enthusiast who wants to stay safe and comfortable during their adventure.