A sash is a wide band of fabric that either secures clothing around your waist or decorates a uniform. The sash on a small child's dress often ties in back with a large bow.
A long dress's sash or a swashbuckling pirate's sash usually ties around the waist, while the sash on a soldier's uniform or a girl scout's outfit fastens across one shoulder. There's also a second meaning of sash — a sliding window frame, the part that holds the glass panes in place. This kind of sash is rooted in the French châssis, or "frame," while the clothing kind of sash comes from the Arabic shash, "muslin cloth."