Bivvy bag (or bivvy sack): Short for bivouac sack, it’s a small, lightweight, person-sized waterproof shelter used as an alternative to a tent.
Down: The soft plumage found under the feathers of a bird. One piece of down is called a plume. Most comes from ducks and geese. For its weight, down is one of the best insulators available – it’s normally more durable and compresses smaller than synthetic insulation but it’s also more expensive and, if it doesn’t have a DWR coating (see below), it loses its insulating power when wet.
Composition (or down ratio): The percentage of down versus feathers. For example, 90/10, means 90 % down and 10 % small feathers. The higher the ratio, the lighter, softer and more compressible the bag will be, and the…