A collision in which kinetic energy remains conserved is called an elastic collision. In such a collision, both the total linear momentum and total kinetic energy of the system before and after the collision remain the same.
In contrast, in an inelastic collision, only momentum is conserved, while kinetic energy is partially or fully converted into other forms of energy (like heat, sound, or deformation energy).
Therefore, if the kinetic energy remains constant, the collision must be elastic.