Wordnet 3.0
NOUN (1)
1. 
 the husband of your mother by a subsequent marriage; 
The Collaborative International Dictionary of English v.0.48:
Stepfather \Step"fa`ther\, n. [AS. ste['o]pfaeder.]
   The husband of one's mother by a subsequent marriage.
   [1913 Webster]
WordNet (r) 3.0 (2006):
stepfather
    n 1: the husband of your mother by a subsequent marriage
Bouvier's Law Dictionary, Revised 6th Ed (1856):
STEP-FATHER. In Latin vitricus, is the husband of one's mother who is not
the father of the person spoken of.