Definition: Hertz (symbol: Hz) is the SI unit of frequency, defined as the number of cycles per second of a periodic phenomenon. One hertz is equivalent to one cycle or event per second.

History/origin: The hertz is named after Heinrich Hertz, a German physicist who proved the existence of electromagnetic waves in the late 19th century. The unit was adopted as a standard unit for frequency in the 20th century.

Current use: Hertz is widely used across various fields to measure frequency, including radio waves, audio signals, computer processing speeds, alternating current (AC) electricity, and any oscillating or repeating phenomenon. It's a fundamental unit in physics, engineering, and technology.