The Collaborative International Dictionary of English v.0.48:
Momentum \Mo*men"tum\, n.; pl. L. Momenta, F. Momentums. [L.
1. (Mech.) The quantity of motion in a moving body, being
always proportioned to the quantity of matter multiplied
by the velocity; impetus.
2. Essential element, or constituent element.
I shall state the several momenta of the distinction
in separate propositions. --Sir W.
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.