Ports and adapters
Ports and adapters (also called hexagonal architecture) is the design pattern that surrounds a core domain model with a layer of ports (interfaces describing what the core needs) and adapters (concrete implementations of those interfaces for specific external systems). The core never depends on adapters; adapters implement the ports the core defines.
The pattern was named by Alistair Cockburn around 2005. The benefit is testability and external-system replaceability: the core domain logic is tested with in-memory adapters; switching from one database to another or one message broker to another is an adapter swap, not a core rewrite. The trade-off is layer proliferation, small applications don't benefit, but applications with many external integrations (databases, message brokers, APIs, file systems) benefit substantially. Hexagonal is sometimes confused with clean architecture and onion architecture; the three are variations on the same dependency-inversion idea with slightly different emphases.