A file that can be processed (executed) by the CPU of the machine through the operating system accordingly to the instructions that it contains.
An executable is a file that causes a computer to perform certain tasks according to the encoded instructions. While an executable file can be hand-coded in machine language, it is far more usual to develop software as source code in a high-level language, or in an assembly language. The high-level source code is compiled into either an executable machine code file or a non-executable machine-code object-file. The equivalent process on assembly language source code is called assembly.
The same source code can be compiled to run under different operating systems, usually with minor operating-system-dependent features inserted in the source code to modify compilation according to the target.
When asking questions whose scope is limited to Windows executables, you can also use pe.
Executable files can be converted to the equivalent source code by the process of decompilation or for object files, disassembly.
From Wikipedia:
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU. However, in a more general sense, a file containing instructions (such as bytecode) for a software interpreter may also be considered executable; even a scripting language source file may therefore be considered executable in this sense. The exact interpretation depends upon the use; while the term often refers only to machine code files, in the context of protection against computer viruses all files which cause potentially hazardous instruction execution, including scripts, are conveniently lumped together.
