PDCA

Plan-Do-Check-Act

Iterative improvement cycle (Deming/Shewhart).

When you'd see it: Continuous-improvement and quality programs. PDCA is the basic loop behind Lean and Six Sigma.

Why it matters: PDCA is a simple improvement cycle: plan a change, do it small, check the result, then act on what you learned. Its power is in repeating it, treating improvement as a loop rather than a one-time project.