The Collaborative International Dictionary of English v.0.48:

Stepmother \Step"moth`er\, n. [AS. ste['o]pm[=o]der.]
   The wife of one's father by a subsequent marriage.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stepmother
    n 1: the wife of your father by a subsequent marriage

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "stepmother":
   dam, foster mother, genetrix, ma, mam, mama, mammy, mater,
   materfamilias, maternal ancestor, matriarch, mom, mommy, mother,
   mummy, stepbrother, stepchild, stepdaughter, stepfather,
   stepsister, stepson, the old woman



Bouvier's Law Dictionary, Revised 6th Ed (1856):

STEP-MOTHER. In Latin noverca, is the wife of one's father, who is not the
mother of the person spoken of.