The speed, or frequency, at which the CPU operates. Usually expressed in GHz.

How fast the "engine" runs. While faster is usually better, keep in mind that you will pay more for faster processors. Speed is specified in Megahertz (millions of cycles per second), and current processors are running in the 300 to 600 Megahertz range. Unless you desperately need it, avoid the very fastest processors; they are expensive and sometimes unstable. Also avoid the very slowest processors, unless you know that your computer can be upgraded to a faster processor if you should so desire.