From The Collaborative International Dictionary of English v.0.48 :
Stepson \Step"son`\, n. [AS. ste['o]psunu.]
A son of one's husband or wife by a former marriage.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
stepson
n 1: the son your spouse by a former marriage
From Bouvier's Law Dictionary, Revised 6th Ed (1856) :
STEP-SON. In Latin privignus, is the son of one's wife, or of one's husband.