stepbrother

from WordNet (r) 3.0 (2006)

stepbrother
    n 1: a brother who has only one parent in common with you [syn:
         {stepbrother}, {half-brother}, {half brother}]

from The Collaborative International Dictionary of English v.0.48

Stepbrother \Step"broth`er\, n.
   A brother by the marriage of one's father with the mother of
   another, or of one's mother with the father of another.
   [1913 Webster]