x86-64 - Wikipedia
https://en.wikipedia.org/wiki/X86-64
x86-64 (also known as x64, x86_64, AMD64 and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mode.
x64 Architecture - Windows drivers | Microsoft Docs
https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/x64-architecture
The x64 architecture is a backwards-compatible extension of x86. The 64-bit registers have names beginning with "r", so for example the 64-bit extension of eax is called rax.