The Collaborative International Dictionary of English v.0.48:
Daughter-in-law \Daugh"ter-in-law`\, n.; pl. Daughters-in-law. The wife of one's son. [1913 Webster]WordNet (r) 3.0 (2006):
daughter-in-law n 1: the wife of your sonBouvier's Law Dictionary, Revised 6th Ed (1856):
DAUGHTER-IN-LAW. In Latin, nurus, is the wife of one's son.