Glossary

TDD (Test-Driven Development)

TDD is a development methodology: write a failing test first, then the minimum code to make it pass, then refactor. The "red → green → refactor" cycle repeats for each small piece of functionality.

The Red-Green-Refactor cycle

Red — write a test for new behaviour. It fails because the code does not exist yet
Green — write the minimum code to make the test pass. Any way works
Refactor — improve the code without changing behaviour. The test stays green

Benefits

New code is 100% covered by definition
Design naturally becomes testable — no tight dependencies
Tests document behaviour

Criticism

Slows initial development. Hard to apply to UI, legacy code, and integrations. Most teams use TDD selectively, not universally.