Clean Architecture
Robert "Uncle Bob" Martin's umbrella for Hexagonal + Onion + DDD — the central rule: dependencies point inward toward the domain.
Formulated in a 2012 blog post, book in 2017. Layers: Entities (domain) → Use Cases → Interface Adapters → Frameworks/Drivers. Inner layers know nothing about outer ones. Testing focus: you can test use cases without a database or web server. Criticised for producing over-engineered Java/C# projects with seven folders just to read a row from the DB. Shines on complex domain logic, feels silly in CRUD apps.