Execution in computer and software engineering is the process by which a computer or a virtual machine performs the instructions of a computer program. The instructions in the program trigger sequences of simple actions on the executing machine. Those actions produce effects according to the semantics of the instructions in the program.
Programs for a computer may execute in a batch process without human interaction, or a user may type commands in an interactive session of an interpreter. In this case the "commands" are simply programs, whose execution is chained together.
The term run is used almost synonymously. A related meaning of both "to run" and "to execute" refers to the specific action of a user starting (or launching or invoking) a program, as in "Please run the application."