From WordNet (r) 3.0 (2006) :
expansion slot
n 1: (computer) a socket in a microcomputer that will accept a
plug-in circuit board; "the PC had three slots for
additional memory" [syn: slot, expansion slot]
From The Free On-line Dictionary of Computing (30 December 2018) :
expansion slot
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)