Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

stepchild
     n : a child of your spouse by a former marriage
     [also: {stepchildren} (pl)]