The Collaborative International Dictionary of English v.0.48:

Modification \Mod`i*fi*ca"tion\, n. [L. modificatio a measuring:
   cf. F. modification. See Modify.]
   1. The act of modifying, or the state of being modified; a
      change; as, the modification of an opinion, or of a
      machine. --Bentley.

   Syn: change, alteration, adjustment.
        [1913 Webster]

   2. Something which has been modified; a modified form or
      condition; state as modified; as, the various
      modifications of light; the latest modification of the
      operating system crashes less frequently.

   Syn: model[8].
        [1913 Webster +PJC]

   3. (Gram.) The alteration of the meaning of a word or phrase
      by another word or phrase; -- usually a restriction of the
      scope of the word modified; as, in the phrase "a billion
      dollars is a relatively small sum to spend on cancer
      research" the modification of small by relatively is
      needed to make the sentence accurate, rather than
      ludicrous.
      [PJC]

WordNet (r) 3.0 (2006):

modification
    n 1: the act of making something different (as e.g. the size of
         a garment) [syn: alteration, modification,
         adjustment]
    2: slightly modified copy; not an exact copy; "a modification of
       last year's model"
    3: the grammatical relation that exists when a word qualifies
       the meaning of the phrase [syn: modification, qualifying,
       limiting]
    4: an event that occurs when something passes from one state or
       phase to another; "the change was intended to increase
       sales"; "this storm is certainly a change for the worse";
       "the neighborhood had undergone few modifications since his
       last visit years ago" [syn: change, alteration,
       modification]