A type of machine used to increase the pressure of a gas or vapour by reducing its volume. It is used to compress a gas or vapour in a vessel, to raise its pressure, and for its transport under pressure through pipelines. Compressors are broadly classified as those that are dynamic and those that are positive displacement in design and operation.

Centrifugal compressors are dynamic-type compressors and consist of various types of rotating impeller used to increase the velocity, which is converted to pressure through a diffuser. They are widely used in the chemical, oil, and gas industries. Axial-flow compressors consist of blades or airfoils mounted on the rotating shaft that compresses the flow along the axis of the shaft rather than radially. They are used for generating compressed gases with a high velocity such as in natural gas compression.

Reciprocating compressors are positive displacement-type compressors and consist of a piston attached to a crankshaft to compress gas that is drawn into a cylinder. Many reciprocating compressors are multistaged and can generate high pressures in excess of 2,000 bar. Rotary screw compressors are also positive displacement-type compressors and consist of two meshing rotors. The compressor on a domestic refrigerator is a hermetically sealed compressor used to compress the evaporated refrigerant vapour for condensation, throttling, and recycling back to the evaporator. They commonly consist of a scroll-type compressor although spindle-type compressors are also used.