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):

gravitation
    n 1: (physics) the force of attraction between all masses in the
         universe; especially the attraction of the earth's mass for
         bodies near its surface; "the more remote the body the less
         the gravity"; "the gravitation between two bodies is
         proportional to the product of their masses and inversely
         proportional to the square of the distance between them";
         "gravitation cannot be held responsible for people falling
         in love"--Albert Einstein [syn: gravity, gravitation,
         gravitational attraction, gravitational force]
    2: movement downward resulting from gravitational attraction;
       "irrigation by gravitation rather than by pumps" [ant:
       levitation]
    3: a figurative movement toward some attraction; "the
       gravitation of the middle class to the suburbs"

The Devil's Dictionary (1881-1906):

GRAVITATION, n.  The tendency of all bodies to approach one another
with a strength proportion to the quantity of matter they contain --
the quantity of matter they contain being ascertained by the strength
of their tendency to approach one another.  This is a lovely and
edifying illustration of how science, having made A the proof of B,
makes B the proof of A.