piston

Any disc or cylinder, circular in cross-section, used in reciprocating engines (steam, petrol, diesel oil) to harness power. Pistons are driven up and down in cylinders by expanding steam or hot gases. They pass on their motion via a connecting rod and crank to a crankshaft, which turns the driving wheels. In a pump or compressor, the role of the piston is reversed, being used to move gases and liquids. See also internal-combustion engine.

piston

On a brass musical instrument, a valve which alters the length of tubing through which air vibrates, changing the pitch. The first valve lowers the pitch a whole tone, the second a semitone, the third three semitones. Most orchestral brass instruments today employ three valves, but sometimes a fourth is added.