Webhook
In plain terms
A user-defined HTTP callback triggered by an event.
Going deeper
A webhook pushes data to a URL you register when something happens, so you don't have to poll for updates. It's event-driven server-to-server communication.
Under the hood
Reverse of polling: the source calls you; needs signature verification and retry handling for reliability.
Examples
Stripe payment events, GitHub push notifications, Slack integrations, CI triggers.
Think of it like...
Like leaving your number so a shop calls you when your order ships, instead of you calling repeatedly.