A gigahertz is one billion (or 109) hertz, a measure of frequency. Each cycle is one nanosecond.

Radio waves sent at gigahertz frequencies usually travel in line of sight. Gigahertz frequencies or microwaves are also used in cellular telephones, microwave ovens, radar and other uses.

See hertz, kilohertz (103Hz), megahertz (106Hz), terahertz (1012Hz), and petahertz (1015Hz).

As of 2003, most of the commonly sold microprocessors work with clocks that have frequencies ranging from one to three gigahertz. Earlier, microchip producers, particularly Intel and AMD made heavy marketing campaigns centered around the "breaking of the gigahertz barrier": this took place in 1999.