Event
Something that "happens" the code wants to react to — click, keypress, network response, timer tick, message arrives. Event-driven programming is built on these.
DOM: element.addEventListener('click', handler). Node.js: EventEmitter. Message bus: Kafka events, Redis pub/sub. Event sourcing: all state changes are stored as immutable events. Event loops (Node.js, browser, libuv) process events from a queue. Event storming is the facilitation technique for domain modeling. CloudEvents (CNCF) is the standardized envelope format for cloud events.