The Collaborative International Dictionary of English v.0.48:

fallback \fall"back\, n.
   1. The act or process of falling back.
      [PJC]

   2. Something or someone to which one resorts as an
      alternative to a failed resource or method.
      [PJC]

WordNet (r) 3.0 (2006):

fallback
    n 1: to break off a military action with an enemy [syn:
         disengagement, fallback, pullout]