The conservation of momentum is a fundamental principle in physics stating that the total momentum of a closed system remains constant if no external forces act on it. Momentum, defined as the product of an object's mass and velocity, is conserved during interactions such as collisions. This means that the total momentum before an event equals the total momentum after the event. For example, in a collision between two billiard balls, the momentum lost by one ball is equal to the momentum gained by the other. This principle applies to various scenarios, including car crashes and rocket launches, making it essential for understanding motion and forces in our universe.