A webhook is a way for a service to tell you about an event as it occurs. You give the service a URL; when something happens, such as a payment succeeding or a form being submitted, it sends an HTTP request to that URL containing details of the event.
It is the opposite of polling, where your system keeps asking whether anything changed. Webhooks are the usual trigger for a workflow in tools like n8n.
Handle them defensively. Reply quickly with a success status and do slow work afterwards. Verify the sender, for example by checking a signature, because anyone who finds the URL can post to it. Expect the same event to be delivered more than once and out of order, so make your handler idempotency-safe, and watch for a race condition when two deliveries arrive together. Senders often retry on failure, which is a form of retry logic.
Related: API, Workflow Automation, Error Handling.