A collision is when two or more moving objects crash into each other. When a soccer ball smashes into a player's shin guards, that's a collision. When two cars hit each other in a parking lot, that's a collision. Even bumping into your friend while racing down the hallway counts as a collision.

The word emphasizes the impact and force of things coming together, usually unexpectedly or violently. Scientists use collision to describe everything from billiard balls striking each other on a pool table to asteroids smashing together in space. In physics class, you might study how objects bounce off each other or stick together after a collision, and how their motion changes based on their speed and mass.

People also use collision metaphorically to describe when different ideas, interests, or plans crash into each other. When your soccer practice and piano recital are scheduled at the same time, you might call it a collision of commitments. When a new technology disrupts an old industry, economists might describe it as a collision between the old way and the new way of doing things.

The key idea behind a collision is the sudden, forceful meeting of things that weren't designed to come together that way.