What is an expansion cord and what is it used for

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Definitions of expansion card on the Web:

A circuit board that plugs into a motherboard, used to add new peripherals to a computer system. Also called an adapter card.
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A special purpose circuit board that can be inserted into your computer to give it additional capabilities. See for example network interface card.
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A circuit board inserted into a slot on the motherboard to enhance the capability of the computer.
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A card that can be fitted into an expansion slot within a computer to improve a part of its performance.
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An 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 to add additional functionality to a computer system. ...
en.wikipedia.org/wiki/Expansion card (http://www.google.com/url?sa=X&start=4&oi=define&q=http://en.wikipedia.org/wiki/Expansion+card&usg=AFQjCNGVVt4TOtAsnui_5zERUSBybagzdg)