Definition
Refactoring is the process of changing the internal structure of code to make it clearer, simpler, or easier to extend, while keeping its external behavior exactly the same. The program does the same thing before and after; only the design improves.
It is a deliberate, ongoing activity, not a one-time cleanup. Code is grown rather than built, and like anything that grows, it accumulates structure that no longer fits and must periodically be reworked.
Why it matters
How it works
Effective refactoring follows two rules. First, do not refactor and add features at the same time; keep the two activities separate so a behavior change is never tangled with a structural change. Second, refactor in small steps, verifying with tests after each one, so a mistake is caught immediately and easily reverted.
Common triggers are duplication, names that no longer reflect intent, code that violates the developer's understanding, or a structure that fights the next change. The presence of good tests is what makes refactoring safe and routine rather than risky.