Conservation of momentum definition
The principle of conservation of momentum states that the total momentum of all objects in a closed system is constant.
A closed system is a system that cannot exchange matter with its surroundings (i.e. the total mass of the system is constant).
Take a gun and a bullet as a closed system. Before shooting, the total momentum is zero. When the bullet is shot (forward momentum), the gun needs to recoil (backward momentum) to keep the system at zero momentum.
The conservation of momentum is closely related to Newton's Third law of motion ("Forces always occur in equal and opposite pairs").
If the sum of all action and reaction forces at one point is zero, the total change of momentum of the system is also zero.