a.out is the default filename for executable output from many compilers, especially in Unix environments. It stands for "assembler output".

a.out is also an old object file format for executables, now superseded by the ELF and COFF formats.

(These two meanings are easily confused, as e.g. the default object file format of a compile/linker might be ELF and used to be a.out in earlier versions, but the default file name is still "a.out".)