The Collaborative International Dictionary of English v.0.48:

Occlusion \Oc*clu"sion\, n. [See Occlude.]
   1. The act of occluding, or the state of being occluded.
      [1913 Webster]

            Constriction and occlusion of the orifice. --Howell.
      [1913 Webster]

   2. (Med.) The transient approximation of the edges of a
      natural opening; imperforation. --Dunglison.
      [1913 Webster]

   Occlusion of gases (Chem. & Physics), the phenomenon of
      absorbing gases, as exhibited by platinum, palladium,
      iron, or charcoal; thus, palladium absorbs, or occludes,
      nearly a thousand times its own volume of hydrogen, and in
      this case a chemical compound seems to be formed.
      [1913 Webster]

WordNet (r) 3.0 (2006):

occlusion
    n 1: closure or blockage (as of a blood vessel)
    2: (meteorology) a composite front when colder air surrounds a
       mass of warm air and forces it aloft [syn: occluded front,
       occlusion]
    3: (dentistry) the normal spatial relation of the teeth when the
       jaws are closed
    4: an obstruction in a pipe or tube; "we had to call a plumber
       to clear out the blockage in the drainpipe" [syn: blockage,
       block, closure, occlusion, stop, stoppage]
    5: the act of blocking [syn: blockage, closure, occlusion]