A processor made by Intel in the early 90's. Was extremely fast for its days. It was essentially a 486 chip that ran at twice the speed of its external bus. You could therefore plug an 80486DX2/66 into a motherboard that used to have an 80486DX/33 in it, and your computer would feel twice as fast. There was also an 80486DX2/50, but it sucked compared to an 80486DX/50, which ran natively at 50Mhz.
Clock multiplying is now a way of life in the computer industry.