A hairpiece is a piece of false hair you wear on your own head so the hair looks thicker, longer, or so a thin spot is covered. It is not a full wig. A wig is a whole head of false hair; a hairpiece is only a part — a topper, a fringe, a small patch at the crown. Though actually, shops and polite talk like hairpiece because it sounds softer than toupee, which is the old joking word for a man’s small cover on a bald spot. People wear hairpieces after hair loss, for a play, or just to add volume for one evening. Some clip in; some are stuck down. The word is ordinary and fairly kind. It is countable: a hairpiece. Do not mix it with a hairgrip (a metal clip) or a hairnet (a mesh). The “piece” is hair, or something that looks like hair.

假髮片、補髮片（整頂才叫假髮 wig；只補一塊才是 hairpiece）