expansion card

The expansion card (also expansion board, adapter card or accessory 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 (the edge connector) that fit exactly into the slot.

Depending on the form factor of the motherboard and case, around one to seven expansion cards can be added to a computer system.

19 or more expansion cards can be installed in backplane systems.
The group of expansion cards that are used for external connectivity, such as network, SAN or modem cards, are commonly referred to as input/output cards (or I/O cards).

The primary purpose of an expansion card is to provide or expand on features not offered by the motherboard.

In the case of expansion of on-board capability, a motherboard may provide a single serial RS232 port or Ethernet port.

An expansion card can be installed to offer multiple RS232 ports or multiple and higher bandwidth Ethernet ports.

In this case, the motherboard provides basic functionality but the expansion card offers additional or enhanced ports.
Compaq) and then the VESA Local Bus Standard, were late 1980s expansion buses that were tied but not exclusive to the 80386 and 80486 CPU bus.

Cardbus, using the PCMCIA connector, is a PCI format that attaches peripherals to the Host PCI Bus via PCI to PCI Bridge.

Apple used a proprietary system with seven 50-pin-slots for Apple II peripheral cards, then later used the NuBus for its Macintosh series until 1995, at which time they switched to a standard PCI Bus.

Generally PCI expansion cards will function on any CPU platform if there is a software driver for that type.
However, the expansion modules attached to these interfaces, though functionally the same as expansion cards, are not technically expansion cards, due to their physical form.

Again, PCjr sidecars are not technically expansion cards, but expansion modules, with the only difference being that the sidecar is an expansion card enclosed in a plastic box (with holes exposing the connectors).