The Collaborative International Dictionary of English v.0.48:

Preclusion \Pre*clu"sion\, n. [L. praeclusio. See Preclude.]
   The act of precluding, or the state of being precluded; a
   shutting out.
   [1913 Webster]

WordNet (r) 3.0 (2006):

preclusion
    n 1: the act of preventing something by anticipating and
         disposing of it effectively [syn: obviation,
         forestalling, preclusion]