1. The process of getting an instrument in tune with itself and/or an external reference pitch.
2. An arrangement of pitches/intervals produced by the open strings of an instrument such as guitar, cello, or violin. Most instruments have a standard tuning (such as E-A-D-G-B-E, low to high, on guitar), but may also be set up for other tunings, known variously as scordatura (in the classical world), “alternate tunings,” “altered tunings,” “drop tunings,” “down tuning,” “open tunings,” and other terms.
3. To optimize a process or device.