A compatibility card is an expansion card

Expansion card

The expansion card in computing is a printed circuit board that can be inserted into an expansion slot of a computer motherboard or backplane to add functionality to a computer system via the expansion bus.One edge of the expansion card holds the contacts that fit exactly into the slot...

 for computers that allows it to have hardware emulation

Hardware emulation

In integrated circuit design, hardware emulation is the process of imitating the behavior of one or more pieces of hardware with another piece of hardware, typically a special purpose emulation system. The emulation model is usually based on RTL source code, which is compiled into the format...

 with another device. The most popular of these were for Macintosh systems that allowed them to emulate Windows PCs via NuBus

NuBus

NuBus is a 32-bit parallel computer bus, originally developed at MIT as a part of the NuMachine workstation project. The first complete implementation of the NuBus and the NuMachine was done by Western Digital for their NuMachine, and for the Lisp Machines Inc. LMI-Lambda. The NuBus was later...

 or PCI. Apple Computer

Apple Computer

Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...

 made many of these cards (including a compatibility card for the Apple IIe

Apple IIe

The Apple IIe is the third model in the Apple II series of personal computers produced by Apple Computer. The e in the name stands for enhanced, referring to the fact that several popular features were now built-in that were only available as upgrades and add-ons in earlier models...

. Later, Orange Micro

Orange Micro

Orange Micro Inc. is a now-defunct computer hardware company which made products for use with Apple computers. The company made a variety of products for many machines, ranging from the Apple II series to the Macintosh line, but is perhaps best known for making PC compatibility cards for the...

 made them, but by the end of the 1990s, greater application availability and the lack of expansion slots on the iMac

IMac

The iMac is a range of all-in-one Macintosh desktop computers built by Apple. It has been the primary part of Apple's consumer desktop offerings since its introduction in 1998, and has evolved through five distinct forms....

 made such cards obsolete. Besides PC compatibility cards, others existed, such as the 3DO Blaster

3DO Blaster

The 3DO Blaster was an add-on designed to allow compatible Windows-based PCs to play games for the 3DO console.It was a full-sized ISA compatibility card, and unlike other such add-ons, it does not emulate a 3DO system, but rather the whole system's logic board is included, with the input and...

for PCs.