The Collaborative International Dictionary of English v.0.48:
Stepdaughter \Step"daugh`ter\, n. [AS. ste['o]pdohtor.] A daughter of one's wife or husband by a former marriage. [1913 Webster]WordNet (r) 3.0 (2006):
stepdaughter n 1: a daughter of your spouse by a former marriageBouvier's Law Dictionary, Revised 6th Ed (1856):
STEP-DAUGHTER. In Latin privigna, is the daughter of one's wife, or of one's husband.