A term used to describe the way a musical instrument plays. Its use is highly varied in the industry, but in a general sense it refers to the playability of an instrument or how well it functions mechanically. It often specifically relates to how much force is required to play the instrument. “Piano action” or the more generic “weighted action” is used to describe some keyboards, for example, where it means that the keyboard itself feels sort of like a piano in its mechanical operation, as opposed to an unweighted synthesizer style action. You have to press harder on a piano to make sound, but this “feel” is very important to players. There is also the “hammer action,” which pertains to a keyboard that actually uses a hammer mechanism internally to more closely simulate the feel of a real piano keyboard. Hammer, weighted, and piano action are terms that tend to be used interchangeably even though they aren’t exactly the same thing. In guitars and basses the action again refers to how the instrument plays, and specifically to how much down force is required on the strings to properly press them against the frets. A lighter or lower (as in strings down closer to the frets) action is considered easier to play, but just like the piano some players don’t like it to be too easy. What constitutes a “good” action is very subjective and each player typically has his own tastes.