The Collaborative International Dictionary of English v.0.48:
Mother-of-pearl \Moth"er-of-pearl`\, n. (Zool.) The hard, iridescent, pearly internal layer of several kinds of shells, esp. of pearl oysters, river mussels, and the abalone shells; nacre. See Pearl. [1913 Webster]WordNet (r) 3.0 (2006):
mother-of-pearl n 1: the iridescent internal layer of a mollusk shell [syn: mother-of-pearl, nacre]