Machines or computer programs that copy how something works, like a flight simulator that copies how a plane works.
Simulators are usually used for training or learning.
Examples:
The pilots use simulators to practice flying.
I want to play with the car simulator.
The doctor uses a simulator to learn how to do surgery.
The kids love the dinosaur simulator at the museum.
In the simulator, you can drive a big truck.
The astronaut uses a simulator to practice going to space.