The 'a.out' file extension, short for "assembler output," historically represents an executable file format primarily used in Unix-like operating systems. While largely superseded by the ELF (Executable and Linkable Format) standard, 'a.out' files still hold historical significance and may occasionally be encountered. These files contain compiled machine code, ready to be executed by the operating system. The format includes sections for program code, data, and symbol tables, enabling the operating system to load the program into memory, resolve external dependencies (libraries), and initiate execution. The 'a.out' format is simpler than ELF, which made it suitable for early Unix systems with limited resources. However, its limitations in terms of features like dynamic linking and support for different architectures led to its eventual replacement. Modern systems rarely use 'a.out' directly, but understanding its role provides valuable context for the evolution of executable file formats.