The Free On-line Dictionary of Computing (30 December 2018):

backplane

    A printed circuit board with slots
   into which other cards are plugged.

   A backplane,is typically just a connector and does not usually
   have many active components on it.  This contrasts with a
   motherboard.

   Designing a backplane
   (http://iec.org/online/tutorials/design_backplane/index.html).

   (2002-09-08)