Final answer:
The statement that momentum remains constant in a collision with no external forces refers to the principle of the conservation of momentum, a fundamental concept in classical mechanics.
Step-by-step explanation:
When two objects collide and there are no external forces acting on them, the statement that the momentum of the system remains constant is a principle known as the conservation of momentum. This principle is a cornerstone of classical mechanics and indicates that the total momentum of a closed system will not change as a result of interactions within the system. An example of this can be seen when two cars collide; car one may slow down, losing momentum, while car two speeds up, gaining momentum. Despite these individual changes, the total momentum of the car system remains constant, assuming all forces between them are internal to the system.