A perfectly elastic collision is a collision between objects during which there is no loss of kinetic energy and energy is fully conserved. No energy is dissipated as heat energy internal to the objects, and no energy is spent on permanently deforming the bodies or radiated away in another way.
Large-scale collisions are typically not perfectly elastic because any macroscopic collision between objects converts some kinetic energy to internal energy or another form of energy. A perfectly elastic collision is an ideal-condition collision and is never fully realized between macroscopic objects.
Some examples of perfectly elastic collisions include the collision of ideal gases, gravitational interactions between planets and satellites, and the collision between two billiard balls. When the balls hit head on, the velocity of one ball is completely transferred to the other ball; it is a perfectly elastic collision because kinetic energy is conserved before and after the impact.