The Collaborative International Dictionary of English v.0.48:

functionalism \functionalism\ n.
   1. a psychology based on the assumption that all mental
      process are useful to an organism in adapting to the
      environment.
      [WordNet 1.5]

   2. any doctrine that stresses utility or purpose.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

functionalism
    n 1: a psychology based on the assumption that all mental
         process are useful to an organism in adapting to the
         environment
    2: any doctrine that stresses utility or purpose