An Acronym for Advanced Micro Devices.
AMD are one of the big player companies in the computer hardware business and the only serious Intel competitor left in the ia32 CPU market. They have also designed an 64bit extension to ia32 called amd64. While their CPUs are the prestigious part of AMD's business, their actual moneymaker is flash memory chips, a market which they rule.
Their ia32 chips are called Athlon and Duron. Because Intel was thumping "true Megahertz" as a measure of the chip's performance, AMD took to labelling these chips with the clockspeed of the Pentium they're roughly as fast as. This is known as "Quantispeed" means that, f.ex, an Athlon XP 1800+ actually runs at 1533Mhz.