An emulator duplicates (provides an emulation of) the functions of one system with a different system, so that the second system behaves like (and appears to be) the first system. This focus on exact reproduction of external behavior is in contrast to simulation, which concerns an abstract model of the system being simulated, often considering internal state.

Emulators in computer science

Emulation refers to the ability of a program or device to imitate another program or device. Many printers, for example, are designed to emulate Hewlett-Packard LaserJet printers because so much software is written for HP printers. By emulating an HP printer, a printer can work with any software written for a real HP printer. Emulation "tricks" the software into believing that a device is really some other device.

A hardware emulator is an emulator which takes the form of a hardware device. Examples include printer emulators inside the ROM of the printer, and FPGA-based hardware emulators.

In a theoretical sense, the Church-Turing thesis implies that any operating environment can be emulated within any other. In practice, it can be quite difficult, particularly when the exact behavior of the system to be emulated is not documented and has to be deduced through reverse engineering. It also says nothing about timing constraints; if the emulator does not perform as quickly as the original hardware, the emulated software may run much more slowly than it would have on the original hardware.