expansion slot

slot in a motherboard into which additional cards can be placed (Computers)

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. One edge of the expansion card holds the contacts (the edge connector) that fit exactly into the slot. They establish the electrical contact between the electronics (mostly integrated circuits) on the card and on the motherboard.
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<hardware> A connector in a computer into which an expansion card can be plugged. The connector supplies power to the card and connects it to the data bus, address bus and control signals of the motherboard.
(1998-06-26)

Noun
1. (computer) a socket in a microcomputer that will accept a plug-in circuit board; "the PC had three slots for additional memory"
(synonym) slot
(hypernym) receptacle
(classification) computer, computing machine, computing device, data processor, electronic computer, information processing system