A waist bag, or fanny pack (American English), belt bag, moon bag, belly bag (American English), or bumbag (British English) is a small fabric pouch worn like a belt around the waist by use of a strap above the hips that is secured usually with some sort of buckle. The straps sometimes have tri-glide slides, making them adjustable in order to fit properly. It can be considered as a purse worn around the waist.

Although traditionally the bag was worn with the pouch at the front, the separate American and British names derive from the fact that they are often worn with the pouch above the buttocks, for which "fanny" and "bum" are respective slang terms in each country. ("Fanny" is somewhat rude term for the vulva in Britain and many other parts of the English-speaking world, and thus avoided in those areas in favor of "bum".)