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]WordNet (r) 3.0 (2006):
stepson n 1: the son your spouse by a former marriageBouvier's Law Dictionary, Revised 6th Ed (1856):
STEP-SON. In Latin privignus, is the son of one's wife, or of one's husband.