A large and usually colorful ribbon or band of material worn around the body, usually either:

Draped diagonally from one shoulder to the opposite hip.
Danbooru post #893699

Wrapped horizontally around the waist.
Danbooru post #8135112

The waist sash may be worn as daily attire, but the shoulder sash is typically worn on ceremonial occasions only. Ceremonial sashes may also be seen in a V-shaped format, draping from both shoulders to the stomach like a large necklace.

An obi is considered a type of sash that wraps around the waist.

Edit

See also