In the context of a guitar, the action is the distance between the strings and the fretboard. A lower action usually makes the guitar easier to play but produces less sustain. An action that is too low may result in buzzing between the string and the frets.

 

An action gauge is a custom ruler that measures action on guitars in inches or millimeters.

 

More generally, 'action' may refer to the mechanism of a piano, organ, or similar instrument, or to the force required to produce a note on an instrument.