The Collaborative International Dictionary of English v.0.48:

Supererogation \Su`per*er`o*ga"tion\, n. [L. supererogatio a
   payment in addition.]
   The act of supererogating; performance of more than duty or
   necessity requires.
   [1913 Webster]

   Works of supererogation (R. C. Ch.), those good deeds
      believed to have been performed by saints, or capable of
      being performed by men, over and above what is required
      for their own salvation.
      [1913 Webster]

WordNet (r) 3.0 (2006):

supererogation
    n 1: an effort above and beyond the call of duty