Rate at which the instructions are processed (clock rate)
Measured in Hertz
1 Hertz - one cycle per second
Processor clock rate measured in MHz
Machines are compared based on their clock speed or number of instructions per second (ISP).
This measure depends on both the number of cycles per second and the mix of instructions executed.
Measure of processor performance is benchmarking.
ZDNet is organization which has a set of useful benchmarks