(FSB) The bus via which a processor communicates with its RAM and chipset; one half of the Dual Independent Bus, the other half being the backside bus. The L2 cache is usually on the FSB, unless it is on the same chip as the processor.

In PCI systems, the PCI bus runs at half the FSB speed.

Intel's Pentium 60 processor used a bus speed and processor speed of 60 MHz. All later processors have used multipliers to increase the internal clock speed while maintaining the same external clock speed, e.g. the Pentium 90 used a 1.5x multiplier. Modern Socket 370 motherboards support multipliers from 4.5x to 8.0x, and FSB speeds from 50 MHz to a proposed 83 MHz standard. These higher speeds may cause problems with some PCI hardware.

Altering the FSB speed and the multiplier ratio are the two main ways of overclocking processors.

Toms Hardware - The Bus Speed Guide.

Toms Hardware - The Overclocking Guide.