The modern-day shape and layout of PC
motherboards. It improves on the previous standard, the
Baby AT form factor, by rotating the orientation of the board 90 degrees. This allows for a more efficient design, with
disk drive cable connectors nearer to the
drive bays and the
CPU closer to the
power supply and cooling fan.
Also see
From AT to BTX: Motherboard Form Factors in Webopedia's "Did
You Know...?" section.