One kilohertz (abbreviated "kHz") is equal to 1,000 hertz. Like hertz, kilohertz is used to measure frequency, or cycles per second. Since one hertz is one cycle per second, one kilohertz is equal to 1,000 cycles per second.
Kilohertz is commonly used to measure the frequencies of sound waves, since the audible spectrum of sound frequencies is between 20 Hz and 20 kHz. For example, middle C (C4) on a piano keyboard produces a frequency of 261.63 Hz. The C key two octaves above middle C (C6) produces a frequency of just over 1 kHz (1,046.5 Hz). Since the sound frequency doubles with each octave, the C7 key produces an audible frequency of just over 2 kHz (2,093 Hz). As you might guess, frequencies above 2 kHz sound very high-pitched.
Sound waves and low frequency radio waves are often measured in kilohertz. Other waves, such as high frequency radio waves, visible light waves, and ultraviolet rays, have much higher frequencies. Therefore, most waves in the electromagnetic spectrum are measured in megahertz, gigahertz, or even greater units of measurements.