What is the name of the first 64-bit version of Windows for x86 processors?

Windows XP Professional x64 Edition was the first 64-bit version of Windows designed for standard x86-compatible processors using x86-64 extensions. Microsoft released it on April 25, 2005, for computers using AMD64 or Intel 64 processors.

This edition is easy to confuse with Windows XP 64-Bit Edition, which targeted Intel’s Itanium architecture. Itanium used the distinct IA-64 instruction set, while Windows XP Professional x64 Edition supported the 64-bit extensions added to the familiar IA-32 or x86 processor family. That distinction makes the stored answer the precise one for x86 processors.

The system shared its underlying codebase with Windows Server 2003 rather than the ordinary 32-bit Windows XP release. It included WOW64, allowing many 32-bit applications to run, but required 64-bit device drivers. Its practical advantage was access to more memory: ordinary 32-bit Windows XP was generally limited to 4 GB, while the x64 edition supported substantially more.