Webhook
A webhook is an automatic notification: when something happens in one system, it sends the event's data to an address in another system.
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Explanation
With an API the other system asks; with a webhook the event speaks for itself, when a new contact is created, for example.
Webhooks are usually signed, so the receiver can check they really came from the sender.
How HelloTeam does it
HelloTeam sends signed webhooks with retries and a delivery log; the automations' webhook step sends and reads replies too.