The Collaborative International Dictionary of English v.0.48:

Stepchild \Step"child`\, n. [AS. ste['o]pcild.]
   1. A bereaved child; one who has lost father or mother.
      [Obs.]
      [1913 Webster]

   2. A son or daughter of one's wife or husband by a former
      marriage.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stepchild
    n 1: a child of your spouse by a former marriage

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "stepchild":
   child, daughter, descendant, foster child, grandchild,
   granddaughter, grandson, heiress, offspring, scion, son,
   son and heir, sonny, stepbrother, stepdaughter, stepfather,
   stepmother, stepsister, stepson