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]

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)