The Collaborative International Dictionary of English v.0.48:

Momentum \Mo*men"tum\, n.; pl. L. Momenta, F. Momentums. [L.
   See Moment.]
   1. (Mech.) The quantity of motion in a moving body, being
      always proportioned to the quantity of matter multiplied
      by the velocity; impetus.
      [1913 Webster]

   2. Essential element, or constituent element.
      [1913 Webster]

            I shall state the several momenta of the distinction
            in separate propositions.             --Sir W.
                                                  Hamilton.
      [1913 Webster]

   3. A property of an activity or course of events, viewed as
      analogous to forward motion or to physical momentum (def.
      1), such that the activity is believed to be able to
      continue moving forward without further application of
      force or effort; -- often used to describe an increase in
      the acquisition of public support for a purpose; as, as,
      the petition drive gained momentum when it was mentioned
      in the newspapers.
      [PJC]

WordNet (r) 3.0 (2006):

momentum
    n 1: an impelling force or strength; "the car's momentum carried
         it off the road" [syn: momentum, impulse]
    2: the product of a body's mass and its velocity; "the momentum
       of the particles was deduced from meteoritic velocities"

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "momentum":
   clout, compulsion, drive, driving force, energy, force, impellent,
   impelling force, impetus, impulse, impulsion, incentive,
   incitement, inertia, irresistible force, moment, motive power,
   power, propulsion, push, strength, thrust