1. The reproduction of the essential features of something; such as, an aid to study or training.
2. The imitation or feigning of something or a false appearance.
3. The construction of a mathematical model to reproduce the characteristics of a phenomenon, system, or process, often using a computer.
4. A computer game which simulates or reproduces a real activity; such as, flying.
5. A broad collection of methods used to study and to analyze the behavior and performance of actual or theoretical systems.

Simulation studies are performed, not with a real-world system, but on a (usually computer-based) model of the system created for the purpose of studying certain system dynamics and characteristics.