What is 'frequency' in an AC circuit, and what unit is it measured in?

FREQUENCY is the number of CYCLES per second of an alternating current (how many times it reverses direction and returns per second), measured in HERTZ (Hz). ASVAB Electronics Information tests AC properties. US household AC is 60 Hz (60 cycles per second); much of the world uses 50 Hz. Higher frequency = more cycles per second. Frequency is important for AC systems, radio, and signals (radio frequencies are in kHz, MHz, GHz). Knowing frequency = cycles per second, measured in hertz, is commonly tested.