A collision is shock, a brutal impact between two objects. A collision can be elastic or inelastic. In physics, a collision involves a transmission of an energy or an impulse from one object to another. There is generally a clearing of a part of the two objects, most of the time under the form of heat or internal distortion.

The most frequent collisions occur in traffic accidents, between two cars, a car and a truck, or a car and a motorbike. Collisions on the roads may also occur against urban elements such as floor lamps, trashes, bus stops ...
A car may also collide a walker.

In order to improve the resistance of vehicles in case of collision, crash-tests are performed. They are done in laboratories, and generally consist in crashing a car or any type of vehicle against a wall.
Dummies are usually used so as to study the consequences of collisions on passengers. Depending upon the results, the vehicles are rated according to an accurate and meticulous scale. The safer is the vehicle, the greater is the mark.

Collisions may occur in air transports as well, whether it is between two planes, or between a plane and the ground. This may happen on the ground, when the planes move on the airport track, or in the skies.

Train mays also collide, and it is most of the time due to a signaling error.