Hertz
From The RadioReference Wiki
The hertz is the unit of frequency. One hertz is defined as one cycle per second. It is named in honor of Heinrich Rudolf Hertz and is abbreviated Hz.
While there are many possible prefixes for the unit Hz, the only ones really relevant to radio are kHz, MHz, and GHz.
kHz = kilohertz = 1,000 hertz
MHz = megahertz = 1,000 kilohertz = 1,000,000 hertz
GHz = gigahertz = 1,000 megahertz = 1,000,000 kilohertz = 1,000,000,000 hertz
See also WikiPedia:Hertz.