file that can be directly run by a computer
An executable is a computer program written by a programmer and compiled by a compiler (See programming). When it is run, it 'executes' (or does) the actions it was programmed to.
An example of a compiler is the GNU C Compiler, or GCC for short. It reads in program code written in the C programming language, and converts it into binary machine code called an executable, so computers can understand it. On Microsoft Windows, these files end with '.exe.' To humans, these files are hard to read. Macs use other file extensions.