In general, the term frequency describes how often an event occurs. In science, frequency describes the number of cycles per second for any type of oscillating or varying current such as sound or radio waves.
Frequencies describe waves, some of which cause audible sounds. The unit used for frequency is the Hertz, which measures cycles or vibrations per second. The shorter the wavelength, the shorter the frequency. A wave that takes 1/10 second to pass will have a frequency of 10 Hz.
Humans can generally hear frequencies in the range of 20 Hz to 20 kHz. Sounds above 20 kHz fall into the ultrasonic range and sounds below 20 Hz fall into the sub-audio range. Animals such as bats, whales and dolphins can produce sounds with frequencies lower than humans can hear.
Frequencies outside the audible range have uses in computer and other electronics applications. Computer processors measure the frequency of clock cycles in megahertz and gigahertz. Other applications of frequency include describing the strength of electromagnetic fields, wireless signals and radio waves.