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

printed circuit board

    (PCB) A thin board to which electronic components
   are fixed by solder.  Component leads and integrated circuit
   pins may pass through holes ("vias") in the board or they may
   be surface mounted, in which case no holes are required
   (though they may still be used to connect different layers).

   The simplest kind of PCB has components and wires on one side
   and interconnections (the printed circuit) on the other.  PCBs
   may have components mounted on both sides and may have many
   internal layers, allowing more connections to fit in the same
   board area.  Boards with internal conductor layers usually
   have "plated-through holes" to improve the electrical
   connection to the internal layers.

   The connections are metal strips (usually copper).  The
   pattern of connections is often produced using photo-resist
   and acid etching.  Boards, especially those for high frequency
   circuits such as modern microprocessors, usually have one or
   more "ground planes" and "power planes" which are large
   areas of copper for greater current carrying ability.

   A computer or other electronic system might be built from
   several PCBs, e.g. processor, memory, graphics controller, disk
   controller etc.  These boards might all plug into a
   motherboard or backplane or be connected by a ribbon
   cable.

   (1995-05-01)