Description

A function or class should have one, and only one, reason to change, meaning it should have a single responsibility or function within a software system.

"Gather together the things that change for the same reasons. Separate those things that change for different reasons." - Robert C. Martin

Begin with a list of difficult design decisions (things that are likely to change) - each module is then designed to hide such a decision from the others.

A variable should mean one thing, and one thing only. It should not mean one thing in one circumstance, and carry a different value from a different domain some other time.