Apparently, the naming for AMD's 64 bit extension of x86 is a mess.

AMD calls it "AMD64 Architecture" http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252,00.html.

There is a web site http://www.x86-64.org dedicated to open source effort on AMD64.

And Linus Torvalds said, "x86-64 it is". http://www.kerneltraffic.org/kernel-traffic/kt20040331_255.html#4

Intel said, "Extended Memory 64 Technology", http://www.intel.com/technology/64bitextensions/index.htm?iid=techtrends+spotlight_64bit

And Microsoft says, "64-bit extended systems", http://www.microsoft.com/windowsserver2003/64bit/extended/default.mspx