An automated HTTP callback that sends data from one system to another when a specified event occurs, without the receiving system needing to poll for updates.
Traditional API integration requires the client to repeatedly ask "is there new data?" (polling). A webhook reverses this: the server notifies the client automatically when something happens. The server sends an HTTP POST request to a URL provided by the client, containing data about the event.
Webhooks are more efficient than polling for event-driven workflows. They reduce unnecessary API calls and enable near-real-time notifications.
Example: An open data portal offers a webhook service. A developer registers a webhook URL for notifications when new datasets are published. Whenever a new dataset is added to the portal, the portal automatically sends an HTTP POST to the developer's URL with details about the new dataset. The developer's application processes the notification and updates its own dataset index — without ever needing to check the portal manually.