The Mach-O (Mach Object) file format is a file format used by macOS, iOS, watchOS, and tvOS for executables, object code, shared libraries, dynamically loaded code, and core dumps. It is a replacement for the older NeXT/SunOS-derived "a.out" format. Mach-O is a flexible and extensible format that supports multiple architectures and object types within a single file. This allows for "fat binaries" that can run on different processor architectures without modification. The format is structured into a header, load commands, and data sections. The header contains information about the file type, architecture, and number of load commands. Load commands specify how the operating system should load and link the file, including information about segments, sections, symbols, and dynamic libraries. The data sections contain the actual code and data of the program. Mach-O files are essential for the execution of applications and system software on Apple platforms.