In music, there are two common meanings for tuning:

Tuning practice, the act of tuning an instrument or voice.
Tuning systems, the various systems of pitches used to tune an instrument, and their theoretical bases.

Tuning is the process of adjusting the pitch of one or many tones from musical instruments to establish typical intervals between these tones. Tuning is usually based on a fixed reference, such as A = 440 Hz. Out of tune refers to a pitch/tone that is either too high (sharp) or too low (flat) in relation to a given reference pitch. While an instrument might be in tune relative to its own range of notes, it may not be considered 'in tune' if it does not match A = 440 Hz (or whatever reference pitch one might be using). Some instruments become 'out of tune' with damage or time and must be readjusted or repaired. Wikipedia, Music Tuning

See Also

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Figure 10.01 - Tuning Forks in Sympathy
Graduation
GRADUATION OF MACHINES
GRADUATION OF SYMPATHETIC MACHINERY
Positive graduating Chladna
sympathetic graduation
tuning fork
Tuning Forks Sympathy