All glossary terms
Cross-cutting
Refactor
Refactoring is changing the internal structure of code without changing its external behaviour. The goal is to make code easier to understand, modify, or test, not to add features. Refactoring under test coverage is low-risk; refactoring without tests is rewriting in disguise.
Martin Fowler's 'Refactoring: Improving the Design of Existing Code' is the canonical reference. The discipline distinguishes refactors (behaviour-preserving) from rewrites (behaviour-changing). A real refactor passes the same tests before and after. The most common refactor anti-pattern: bundling a refactor with a feature change, then claiming any regression was 'just the refactor'. Making both changes individually verifiable is the point.