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Longman Dictionary of Contemporary English
expansion card
▪ But the user must be patient when learning the ins and outs of an expansion card.
▪ Choice of an expansion card, with a particular processor, will be determined largely by intended application for the device.
▪ However, the 3.5-inch cage projects into the expansion card space so you can only fit five full-length cards.
▪ It ha six free 16-bit slots, but the power switch cable blocks access to one full-length expansion card.
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 electrical connector, or expansion slot on a computer motherboard, backplane or riser card to add functionality to a computer system via the expansion bus.

An expansion bus is a computer bus which moves information between the internal hardware of a computer system (including the CPU and RAM) and peripheral devices. It is a collection of wires and protocols that allows for the expansion of a computer.