Webhook

A webhook is a way for one service to notify another in real time. Instead of a workflow polling 'anything new yet?' on a loop, the source system posts a small message the instant an event occurs (a form is submitted, a payment clears), and that message starts the work.

It is the common entry point for automations that need to react immediately. Because a webhook is an open door, it has to be secured and rate-limited so only legitimate events get through.