Source : Webster's Revised Unabridged Dictionary (1913)

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
      into the velocity; impetus.

   2. Essential element, or constituent element.

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