- SI unit of frequency, equal to one oscillation per second
- named after Heinrich Hertz, who first demonstrated electromagnetic waves
- audible range: 20 Hz to 20 kHz
- mains electricity: 50 Hz (most of the world) or 60 Hz (Americas, parts of Asia)
- radio: kHz to GHz; Wi-Fi operates at 2.4 GHz and 5 GHz
- processor clock speeds measured in GHz
- musical standard pitch: A4 = 440 Hz
- related to wavelength by f = v / lambda
- refresh rate of displays measured in Hz: 60, 120, 144 Hz common