http://en.wikipedia.org/wiki/X86-64 says "Prior to launch, "x86-64" and "x86_64" were used to refer to the instruction set. Upon release, AMD named it AMD64." and refers to https://wiki.debian.org/DebianAMD64Faq which says ""AMD64" is the name chosen by AMD for their 64-bit extension to the Intel x86 instruction set. Before release, it was called "x86-64" or "x86_64", and some distributions still use these names."

I, myself, had an AMD Opteron machine on which I ran an x86-64 build, with that name for the architecture, of SUSE Linux before Intel released their EM64T.

Here's a source to back up maggit's Wikipedia citations:

http://www.amd.com/us/press-releases/Pages/Press_Release_715...

Dated 8/10/2000.