Collision
A collision is an interaction between two or more patterns that are initially disjoint and non-interacting.
Usually, each of participating patterns is a common, relatively simple object or reaction. It is often possible to rewind a collision back in time by finding predecessors for each of the participating patterns, while preserving the moment of first interaction.
Frequently discussed types of collisions include collisions between spaceships (e.g. glider syntheses), collisions between a spaceship and a stationary pattern (e.g. one-glider seeds), collisions between several spaceships and a stationary pattern (e.g. synthesis components).
Less frequently, "collision" may refer to an arbitrary pattern interaction (for example, the interaction between queen bee shuttles in the Gosper's glider gun).