To "bivouac" is a general term used for setting up a make do campsite ie. "While on the march we bivouaced by the river".
A "bivy sac" is a small bag that fits over, or replaces, a sleeping bag and is used instead of a tent as an emergency shelter while climbing. You don't want to haul a tent, but may get caught out overnight on a route, so you take a bivy sac.
Soldiers usually don't carry bivy sacs. For emergency shelter they have waterproof ponchos that can snap together to make small shelters.
Of course you can use a bivy sac anywhere one wants lightweight shelter and it really doesn't matter in general if someone calls their tent a bivy, except in a discussion where people are talking about the size of shelters related to comfort and weight. In that case it just causes confusion.
Last edited by Happy Feet; 06-25-19 at 08:42 AM.