The Collaborative International Dictionary of English v.0.48:

Gravitation \Grav"i*ta"tion\, n. [Cf. F. gravitation. See
   Gravity.]
   1. The act of gravitating.
      [1913 Webster]

   2. (Pysics) That species of attraction or force by which all
      bodies or particles of matter in the universe tend toward
      each other; called also attraction of gravitation,
      universal gravitation, and universal gravity. See
      Attraction, and Weight.
      [1913 Webster]

   Law of gravitation, that law in accordance with which
      gravitation acts, namely, that every two bodies or
      portions of matter in the universe attract each other with
      a force proportional directly to the quantity of matter
      they contain, and inversely to the squares of their
      distances.
      [1913 Webster]

WordNet (r) 3.0 (2006):

law of gravitation
    n 1: (physics) the law that states any two bodies attract each
         other with a force that is directly proportional to the
         product of their masses and inversely proportional to the
         square of the distance between them [syn: law of
         gravitation, Newton's law of gravitation]