The term 'x86_64' (also known as AMD64 or Intel 64) is not a standard file extension like '.exe' or '.elf'. Instead, it is an architectural specification referring to the 64-bit extension to the x86 instruction set architecture, primarily used by modern desktop and server CPUs from Intel and AMD. When encountered in a file context, such as in a filename or a directory structure (e.g., 'program_x86_64'), it signifies that the associated file is a compiled binary or executable specifically built to run on a system utilizing the x86-64 instruction set. These executables contain machine code directly understandable by the 64-bit processor. The actual file format of the executable will depend on the operating system; for Linux, it is typically ELF (Executable and Linkable Format); for Windows, it is PE (Portable Executable); and for macOS, it is Mach-O. Therefore, 'x86_64' describes the target CPU architecture for the machine code contained within the executable file.