In computer hardware, this is the factor by which the processor runs above the front side bus speed. In a 800 MHz CPU, with a 100 MHz bus, the multiplier is 8.

In an effort to curb fraud-by-overclocking (where a retailer buys, say, 200 MHz chips, overclocks them to 300 MHz, then sells systems with these chips as "normal" 300 MHz), Intel locked the multipliers on their chips starting with the P1 MMX's. This meant that the chip "knew" what multiplier it was supposed to run at, and if you told the motherboard to send it something else it just wouldn't boot. As an unhappy side effect (which led to many, many conspiracy theories in the overclocker community), overclocking these and all the rest of Intel's line was limited to overclocking the front side bus -- a task that is both very reliant on good hardware and very hard on important stuff like hard disk controllers. There were some bright spots -- the legendary Celeron 300A, which almost without fail could be overclocked to 450 MHz by upping the fsb to 100MHz. AMD has also locked the multipliers on their chips. However, in order to appeal to the enthusiast / overclocker community, they have made it playfully easy to unlock the processors. Check out my Duron writeup for a how-to.