Pronunciation: (mō-men'tum),[key] —n., —pl.-taPronunciation: (-tu),[key]-tums. 1. force or speed of movement; impetus, as of a physical object or course of events: The car gained momentum going downhill. Her career lost momentum after two unsuccessful films. 2. Mech.a quantity expressing the motion of a body or system, equal to the product of the mass of a body and its velocity, and for a system equal to the vector sum of the products of mass and velocity of each particle in the system.
3. Philos.moment (def. 7).