For an oscillating or varying current, frequency is the number of complete cycles per second in alternating current direction. The standard unit of frequency is the hertz, abbreviated Hz. If a current completes one cycle per second, then the frequency is 1 Hz; 60 cycles per second equals 60 Hz (the standard alternating-current utility frequency in some countries).
Larger units of frequency include the kilohertz (kHz) representing thousands (1,000's) of cycles per second, the megahertz (MHz) representing millions (1,000,000's) of cycles per second, and the gigahertz (GHz) representing billions (1,000,000,000's) of cycles per second. Occasionally the terahertz (THz) is used; 1 THz = 1,000,000,000,000 cycles per second. Note that these prefixes represent specific powers of 10, in contrast to the prefixes for multiples of bytes, which represent specific powers of 2.
Computer clock speed is generally specified in megahertz and, more recently, in gigahertz.
Frequency is important in wireless communications, where the frequency of a signal is mathematically related to the wavelength. If f is the frequency of an electromagnetic field in free space as measured in megahertz, and w is the wavelength as measured in meters, then
w = 300/f
and conversely
f = 300/w
Also see the definitions of signal and electromagnetic radiation spectrum.