1. A statement or explanation of something that must be done, often given by someone in authority: "The instruction of the recruits was the responsibility of the first sergeant.".
2. A spoken or written statement of what must be done; especially, delivered formally, with official authority, or as an order.
3. Printed information explaining how to use or do something.
4. A line of code written as part of a computer program.
5. The activities of educating or instructing or teaching, or the profession of teaching; including activities that impart knowledge or skills: "The instructions for studying the concepts of chemistry were very clear."

In computer science, an expression which is a source program that resembles an instruction in form, but which does not have a corresponding machine instruction in the object program, and is directed to the assembler or compiler.