Momentum

In physics, collision does not means that one particle strike another particle. Infect, two particles may not even touch each other & may still said to be colliding.
The necessary requirements of collision are
(i) A large force for a relatively short time (i.e., an impulse) acts on each colliding particle.
(ii) The motion of the particles (at least one of the particle) is changed abruptly.
(iii) The total momentum (as also the total energy) of particles remains conserved.