A collision is when two or more things smash together. You shouldn't give yourself a pedicure while driving because it might result in a collision, wrecking your car and spoiling your toenail polish to boot.
Collision comes from the Latin col-, "together," and lædĕre, "to hurt by striking." When a train hits a car, that's a collision. An asteroid hurtling toward earth is on a "collision course" with our planet. Collision also works for abstract conflicts that get violent or hurt your brain somehow, like the collision between two politicians during a debate, or the collision of colors in your outfit.