Abbreviation for gigahertz. One GHz represents 1 billion cycles per second. The speed of microprocessors, called the clock speed, often is measured in gigahertz. For example, a microprocessor that runs at 200 GHz executes 200 billion cycles per second. Each computer instruction requires a fixed number of cycles, so the clock speed determines how many instructions per second the microprocessor can execute. To a large degree, this controls how powerful the microprocessor is.