collision
A collision is what happens when two objects meet, exert strong forces on each other for a brief moment, and then move apart with changed motion. Billiard balls clicking, a bat meeting a ball, cars crunching, even two galaxies passing through each other — all are collisions. The defining feature is that the interaction is intense but short.
Because the contact is so brief, the forces the objects exert on each other are huge compared with any outside forces (friction, gravity) acting during that instant. So to a very good approximation the pair is an isolated system, and the total momentum is conserved: the sum of m times v for both objects is the same just before and just after. What may or may not be conserved is kinetic energy — and that is exactly how we sort collisions into types.
Physicists classify collisions by what happens to kinetic energy: elastic (kinetic energy conserved), inelastic (some kinetic energy lost to heat, sound, and deformation), and perfectly inelastic (the objects stick together, losing the most kinetic energy that momentum conservation still allows). Momentum, though, is conserved in all three.
Two cars, 1000 kg each, meet head-on at 20 m/s and crumple to a stop, locked together. Before: momenta +20000 and -20000 kg m/s, total zero. After: the wreck is motionless, total zero — momentum is conserved. But all the kinetic energy (two lots of 1/2 times 1000 times 20^2 = 200000 J) vanished into bent metal, heat, and noise. That is an inelastic collision.
Momentum is conserved even in a wreck; the kinetic energy is not.
A common mistake is to assume that because two objects hit hard, energy must be conserved. It is momentum that is (almost) always conserved in a collision; kinetic energy is conserved only in the idealized elastic case.