Executable
A file the OS can load and run directly — contains machine code, metadata, and info on which libraries it needs.
Format per OS: ELF (Linux/BSD), Mach-O (macOS), PE/COFF (Windows), WASM (web). Contents: header, sections (.text, .data, .bss), symbol table, relocations. The file command identifies the format. Stripped binary = without debug symbols. Statically linked binary bundles all libraries.