The Collaborative International Dictionary of English v.0.48:

malapropism \mal"a*prop*ism\, n. [From Mrs. Malaprop, a
   character in Sheridan's drama, " The Rivals," who makes
   amusing blunders in her use of words. See Malapropos.]
   A grotesque misuse of a word; a word so used.
   [1913 Webster]

WordNet (r) 3.0 (2006):

malapropism
    n 1: the unintentional misuse of a word by confusion with one
         that sounds similar [syn: malapropism, malaprop]