Physical quantity that is equal to a body’s mass times its velocity (in special relativity: its relativistic mass times its velocity).

What makes momentum a useful quantity is that it is conserved – if several bodies interact, the sum of their momenta before and after the interaction is the same. Momentum is neither created nor destroyed, merely passed on from one body to another.

In general relativity, momentum is one of a number of quantities (like mass and energy) that acts as a source of gravity.