The Collaborative International Dictionary of English v.0.48:

Impaction \Im*pac"tion\, n. [L. impactio a striking : cf. F.
   impaction.]
   1. (Surg.) The driving of one fragment of bone into another
      so that the fragments are not movable upon each other; as,
      impaction of the skull or of the hip.
      [1913 Webster]

   2. An immovable packing; (Med.), a lodgment of something in a
      strait or passage of the body; as, impaction of the fetal
      head in the strait of the pelvis; impaction of food or
      feces in the intestines of man or beast.
      [1913 Webster]

WordNet (r) 3.0 (2006):

impaction
    n 1: the condition of being pressed closely together and firmly
         fixed
    2: a disorder in which feces are impacted in the lower colon
    3: a disorder in which a tooth is so crowded in its socket that
       it cannot erupt normally [syn: impaction, impacted tooth]
    4: a sharp collision produced by striking or dashing against
       something [syn: impingement, impaction]