A messaging pattern where senders (publishers) broadcast messages to a topic without knowing who receives them, and receivers (subscribers) listen to topics they're interested in. Like a radio station broadcasting to anyone tuned in, pub/sub decouples producers from consumers. AWS SNS, Azure Service Bus Topics, Google Cloud Pub/Sub, and OCI Notifications all implement this pattern.
An e-commerce system publishes 'order placed' events to a topic. Multiple subscribers (inventory, shipping, email notification) each receive the message and act independently without the order service knowing about them.