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Longman Dictionary of Contemporary English
expansion slot
noun
EXAMPLES FROM CORPUS
▪ It is very important to have the greatest possible support around the expansion slot area.
▪ The card fits into a free expansion slot on the motherboard.
Wiktionary
expansion slot

n. (context computing English) A slot for additional hardware, cards, etc. for the purpose of expanding a computer system.

WordNet
expansion slot

n. (computer) a socket in a microcomputer that will accept a plug-in circuit board; "the PC had three slots for additional memory" [syn: slot]