Actor model
Concurrency model where "actors" are autonomous units that communicate exclusively via asynchronous messages — no shared state, no locks.
Carl Hewitt et al. (1973). Each actor has a mailbox, handles one message at a time, can spawn new actors and change its own behaviour. Scales horizontally across processes and machines — a crashed actor doesn't take others with it ("let it crash").
Implementations: Erlang/OTP (genesis), Elixir, Akka (JVM), Orleans (.NET), Pony, CAF (C++). Powers WhatsApp, Discord, Klarna, RabbitMQ. Modern descendant: Rust's tokio::actor pattern and CSP-inspired goroutines.