A.out

a.out is a file format used in older versions of Unix-like computer operating systems for executables, object code, and, in later systems, shared libraries. This is an abbreviated form of "assembler output", the filename of the output of Ken Thompson's PDP-7 assembler. The term was subsequently applied to the format of the resulting file to contrast with other formats for object code.
"a.out" remains the default output file name for executables created by certain compilers and linkers when no output name is specified, even though the created files actually are not in the a.out format.
History.
An "a.out" format for the PDP-7, similar to the "a.out" format used on the PDP-11, appeared in the first edition of UNIX.
It was superseded by the COFF format in AT&T Unix System V, which was in turn superseded by the ELF format in System V Release 4.
Though Berkeley Unix continued using the "a.out" format for some time, modern BSD-systems have since switched to ELF. NetBSD/i386 switched formally from "a.out" to ELF in its 1.5 release (Dec 2000). FreeBSD/i386 switched to ELF during the 2.2 to 3.0 transition (Oct 1998).
MINIX 3 switched to ELF in the 3.2.0 release.
Linux also used "a.out" until kernel 1.2 (March 1995), when it was superseded by ELF for that platform as well.
(ELF support was added in the experimental 1.1.52 kernel.) Linux's transition to ELF was more or less forced due to the complex nature of building "a.out" shared libraries on that platform, which included the need to register the virtual address space at which the library was located with a central authority, as the "a.out" ld.so in Linux was unable to relocate shared libraries.
The various BSD flavors were able to continue using "a.out" binaries long after Linux was forced to switch to ELF, due to the somewhat more flexible nature of the BSD "a.out" format compared to that of Linux. The a.out file format on Linux was deprecated with the release of the 5.1 Linux kernel.
Use.
Debugging.
The "a.out" support for debug information is done through the use of special entries in the symbol table called stabs. The stabs format has also been used in many COFF and ELF variations.