What does "castanets" mean?

Castanets: Two small, concave shells of ivory or hard wood, shaped like spoons, fastened to the thumb, and beaten together with the middle finger; -- used by the Spaniards and Moors as an accompaniment to their dance and guitars. Note: The singular, castanet, is used of one of the pair, or, sometimes, of the pair forming the instrument. The dancer, holding a castanet in each hand, rattles then to the motion of his feet. Moore (Encyc. of Music).