From 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]
From WordNet (r) 3.0 (2006) :
stepdaughter
n 1: a daughter of your spouse by a former marriage
From 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.