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 marriage

Bouvier's Law Dictionary, Revised 6th Ed (1856):

STEP-DAUGHTER. In Latin privigna, is the daughter of one's wife, or of one's
husband.