Pitch (music)
In music, the pitch of a note means how high or low the note sounds. For example, a violin usually has a higher pitch than a string bass. Pitch mostly depends on the frequency of the sound — how fast the air vibrates back and forth. In physics, frequency is measured in a unit called Hertz.  A note that is vibrating at 261 Hz will be caused by sound waves that vibrate at 261 times a second.  This will be Middle C on the piano.