Supercharger (also known as a blower) is an air compressor used to force air into the combustion chamber(s) of an internal combustion engine at pressures higher than would otherwise be the case. Any device which does this is a supercharger.
The additional mass of oxygen-containing air that is forced into the engine improves its volumetric efficiency which allows it to burn more fuel - which in turn makes it produce more power. A supercharger can be powered mechanically by belt-, gear- or chain-drive from the engine's crankshaft. It can also be driven by a gas turbine powered by the exhaust gasses from the engine. Turbine-driven superchargers are correctly referred to as turbo-superchargers - or more commonly as turbochargers.