(MegaHertZ) One million cycles per second. It is used to measure the transmission speed of electronic devices, including channels, buses and the computer's internal clock. A one-megahertz clock (1 MHz) means some number of bits (16, 32, 64, etc.) are manipulated one million times per second. A one-gigahertz clock (1 GHz) means one billion times.

MHz and GHz are used to measure the speed of the CPU. For example, a 1.6 GHz computer processes data internally (calculates, compares, etc.) twice as fast as an 800 MHz machine. However, the doubled clock speed of the CPU does not mean twice as much finished work gets done in the same time frame. Internal cache design, bus speed, disk speed, network speed and software design all contribute to the computer's overall processing speed and performance (overall throughput).

Users are often dismayed to find that they only obtain incremental improvements after purchasing a computer rated much faster than their old one. In addition, newer versions of software are often less efficient than previous ones. A faster computer is often required just to maintain the same performance level as the old software. See MIPS, Hertz and space/time.

MHz and GHz Are the Heartbeat
When referencing CPU speed, the megahertz and gigahertz ratings are really the heartbeat of the computer, providing the raw, steady pulses that energize the circuits. If you know German, it's easy to remember this. The word "Herz," pronounced "hayrtz," means heart. This was a coincidence, because in 1883, Heinrich Hertz identified electromagnetic waves.