A frequency counter is an electronic instrument used to measure the frequency of a periodic signal. It works by counting the number of cycles of the input signal within a specific time period, usually expressed in Hertz (Hz). Frequency counters are commonly used in various fields such as telecommunications, engineering, physics, and electronics to accurately determine the frequency of signals generated by oscillators, radios, and other electronic devices. They come in various forms ranging from handheld portable units to larger benchtop instruments with advanced features for precise measurements.