Extensions, reflecting an enhancement by the same name to the architecture of Intel
Pentium processors that improved the performance of multimedia applications. "
[Apologies for the formatting; that's Acrobat's fault, not mine.]
It's now available if you're interested in reading it. So far, it's pretty dry, but interesting. The above quote amused me because - if I remember correctly - the Pentium didn't ship until late 1993, and first Pentium with MMX (which presumably stood for multimedia extensions) didn't ship until early 1997, which makes the reasoning given here for the name pretty specious IMHO.