Stands for "Front-Side Bus."
An FSB was the interface between a computer's CPU and its memory controller. All activity between the CPU and the rest of the computer traveled across the FSB, so it was also called the "system bus." FSBs were common in computers in the 1990s and 2000s, but they became obsolete when processor makers started integrating the memory controller directly into the CPU.
A computer's CPU used the FSB to connect to a pair of chips, called the northbridge and the southbridge and referred to collectively as a chipset. The CPU first connected to the northbridge, which then communicated with the system RAM and high-speed expansion slots (like AGP and PCI Express). The northbridge used an internal bus to connect to the southbridge, which communicated with the rest of the computer — the PCI bus, storage devices, USB controller, and other I/O components.
The speed of the front-side bus — how many times it transfers data between the CPU and the system memory — was measured in megahertz. The CPU's speed was linked directly to the speed of the FSB and was the result of the bus speed multiplied by a clock multiplier set in the system's BIOS. For example, a 2.4 GHz CPU using a 400 MHz bus used a clock multiplier of 6.0. However, larger clock multipliers made computers less efficient because the processor often had to wait for data to transmit over the system bus. Eventually, CPU designers like Intel and AMD created new processors that integrated the memory controller directly into the CPU, bypassing the need for the northbridge and the front-side bus.