One of the most powerful laws in physics is the law of momentum conservation. The law of momentum conservation can be stated as follows.For a collision occurring between object 1 and object 2 in an isolated system, the total momentum of the two objects before the collision is equal to the total momentum of the two objects after the collision. That is, the momentum lost by object 1 is equal to the momentum gained by object 2.The above statement tells us that the total momentum of a collection of objects (a system) is conserved - that is, the total amount of momentum is a constant or unchanging value.

The quantity of motion of a moving body measured as a product of its mass and velocity

Conservation of momentum

the principle that the total linear  or angular momentum in any  isolated system is constant, provided that no external force is applied