The Collaborative International Dictionary of English v.0.48:

Impact \Im*pact"\, v. t. [imp. & p. p. Impacted; p. pr. & vb.
   n. Impacting.] [L. impactus, p. p. of impingere to push,
   strike against. See Impinge.]
   1. To drive close; to press firmly together: to wedge into a
      place. --Woodward.
      [1913 Webster]

   2. To affect or influence, especially in a significant or
      undesirable manner; as, budget cuts impacted the entire
      research program; the fish populations were adversely
      impacted by pollution.
      [PJC]

   3. To collide forcefully with; to strike.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Impact \Im"pact\, n.
   1. Contact or impression by touch; collision; forcible
      contact; force communicated.
      [1913 Webster]

            The quarrel, by that impact driven.   --Southey.
      [1913 Webster]

   2. (Mech.) The single instantaneous stroke of a body in
      motion against another either in motion or at rest.
      [1913 Webster]

WordNet (r) 3.0 (2006):

impact
    n 1: the striking of one body against another
    2: a forceful consequence; a strong effect; "the book had an
       important impact on my thinking"; "the book packs a wallop"
       [syn: impact, wallop]
    3: influencing strongly; "they resented the impingement of
       American values on European culture" [syn: impingement,
       encroachment, impact]
    4: the violent interaction of individuals or groups entering
       into combat; "the armies met in the shock of battle" [syn:
       shock, impact]
    v 1: press or wedge together; pack together
    2: have an effect upon; "Will the new rules affect me?" [syn:
       affect, impact, bear upon, bear on, touch on,
       touch]