From Physics Simulation Wiki

Determinism

A physics engine is deterministic when it behaves the same each time it is "replayed" from its original state. Different applications have different requirements for determinism.

• Action replay as in sports games, e.g. watching a goal scored from various angles.

• Implementing network play by duplicating the physics engine on all computers involved in the simulation. The simulations drift out of sync if the engine is not deterministic.

• Even with a more conventional client/server model, it is not uncommon to do some physics on the client to help hide the network latency.

See also Framerate Independence and Canonical Game Loop.