The Executable and Linkable Format (ELF) is a common standard in computing for executables and object code. First published in the Tool Interface Standard and the System V Application Binary Interface, it was quickly accepted among different vendors of UNIX systems.
Today the ELF format has replaced the proprietary (or sometimes just platform-specific) executable formats in the Linux, Solaris, Irix, and some BSD operating systems.
Other object code file formats are a.out and COFF; ELF could be considered a "competitor" to those.