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