"Bivvy"..."bothy"...."vindsäck"... In my opinion the difference is mainly semantics.

In all cases we are talking about a simple, extremely lightweight, one layer, wind and rain proof shelter. They protect you from the wind, keep you dry, and somewhat hold in body heat. North Americans most often seem to call them "bivvys" while Brits usually use the term "bothy".

There are a variety of designs. They can be made in a pattern somewhat like a sleeping bag (one or two person), or more like a big sack for two or more people (with or without a floor). Climbers often have webbing loops sewn onto the outside of theirs, to attach to anchors so they don't slide off the ledge in the middle of the night. Some have windows or air vents.

Whatever terminology you prefer, they are a handy thing to have when you don't want to carry a tent and sleeping bag, but might have to spend a night out in severe conditions.