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"

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "gravitation":
   G, G suit, adduction, affinity, allurement, apogeotropism,
   attractance, attraction, attractiveness, attractivity, capillarity,
   capillary attraction, cascade, cataract, centripetal force, chute,
   collapse, comedown, crash, debacle, declension, declination,
   defluxion, descending, descension, descent, down, downbend,
   downcome, downcurve, downfall, downflow, downgrade, downpour,
   downrush, downtrend, downturn, downward trend, drag, draw, drop,
   dropping, fall, falling, geotropism, graviton, gravity,
   inclination, magnetism, mass, mutual attraction, plummeting,
   pounce, pull, pulling power, rapids, specific gravity, stoop,
   swoop, sympathy, traction, tug, waterfall



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.