The term 'x64' is not a standard, universally recognized file extension in the same way as '.exe' or '.pdf'. Instead, 'x64' is overwhelmingly used as an architectural identifier, specifically referring to the 64-bit instruction set architecture developed by AMD and later adopted by Intel (also known as AMD64 or Intel 64). When encountered in a file context, it usually implies that the associated file—which might have an extension like .dll, .exe, .so, or .dylib—is compiled specifically for the 64-bit version of the operating system. For example, a folder structure might contain 'program_x86' (for 32-bit) and 'program_x64' (for 64-bit). If a file itself is named 'something.x64', it is highly non-standard, but contextually suggests a 64-bit executable or library file. These files contain machine code instructions designed to run on processors supporting the x86-64 instruction set, allowing access to larger amounts of memory (over 4GB) and utilizing modern CPU features unavailable to 32-bit applications. Therefore, the 'format' is defined by the underlying binary structure (like PE for Windows or ELF for Linux) tailored for the 64-bit environment.