Megaherz is a unit of frequency equal to one million cycles per second. Hertz measure cycles per second, and Mega means one million. Thus five-megahertz is five million cycles per second. When used in the context of radio, MHz refers to the number of oscillations of electromagnetic radiation per second. Several parts of the radio spectrum fall into the MHz range:
LF (Low Frequency) 0.03 – 0.3 MHz
MF (Medium Frequency) 0.3-3 MHz
HF (High Frequency) 3-30 MHz
VHF (Very High Frequency) 30 – 300 MHz
UHF (Ultra High Frequency) 300 – 3000 MHz
The HF, VHF, UHF references are something of a misnomer: most radio communications today occur at higher frequencies due to congestion in the lower frequency bands. Experts in the field of radio communications classify these other categories of spectrum by bands. The names of these bands are idiosyncratic, but are used often in radio communications.
Megahertz in computing:
When referring to a computer a computer processor, MHz is short for Mega Hertz and is one million Hertz. Most CPUs made between 1974 and 2000 were labeled in terms of megahertz (though modern computers have processor speeds in the gigahertz (GHz). The number of megahertz refers to the frequency of the CPU’s master clock signal (“clock speed”).
For example, a microprocessor that runs at 200 MHz executes 200 million 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. Another chief factor in determining a microprocessor’s power is its data width (that is, how many bits it can manipulate at one time).
In addition to microprocessors, the speeds of buses and interfaces are also measured in MHz.