AMD's AMD64 (previously known as x86-64) architecture, a 64-bit version of the x86 architecture (used in AMD's Athlon 64, Opteron, Sempron, and Turion 64 CPUs).
Intel now uses the same instruction set in newer Pentium 4 and Xeon processors and in Core 2 processors, calling it Intel 64 (previously EM64T, originally IA-32e). (Software manufacturers Microsoft and Sun call this instruction set architecture "x64".)