The hertz (symbol: Hz, definition: 1/s) is a unit derived from time which measures frequency in the International System of Units (SI). Frequency is how often something happens. A frequency of 1 hertz means that something happens once a second. The note Middle C (the C in the middle of the piano) is 261.65 (262) Hz. This means there are 262 vibrations every second, which is heard as the note Middle C, when that piano key is played.