A compressor is a mechanical device that increases the pressure of a gas
by reducing its volume. An air compressor is a specific type of gas
compressor.
Compressors are similar to pumps: both increase the pressure on a fluid and
both can transport the fluid through a pipe. The main distinction is that the
focus of a compressor is to change the density or volume of the fluid, which
is mostly only achievable on gases. Gases are compressible, while liquids are
relatively incompressible, so compressors are rarely used for liquids. The
main action of a pump is to pressurize and transport liquids.
Many compressors can be staged, that is, the fluid is compressed several times
in steps or stages, to increase discharge pressure.