Webhook
A mechanism where one system automatically notifies another when something happens, by sending an HTTP request to a URL you registered. Instead of polling for updates, the event comes to you.
A webhook is an automatic notification between systems. You register one of your URLs with the source service; when an event happens there (a message arrives, a payment is approved, a form is submitted), the service makes an HTTP request (usually a `POST` with a JSON payload describing the event) to that URL, immediately.
The alternative is *polling*: your system asking every so often "anything new?". Webhooks remove that waste: no repeated questions, the data arrives as soon as it exists. That is why they are the backbone of integrations and automations: platforms like Zapier essentially work by receiving and firing webhooks between hundreds of apps.
Concrete example: a WhatsApp chatbot. The WhatsApp API receives the customer's message and immediately fires a webhook to the bot's server, which processes the text and replies. Without webhooks, the bot would have to query the API every few seconds to check for new messages.
In practice, webhook receivers need to handle two things: validating the origin (signatures or tokens, so forged requests are rejected) and responding quickly (acknowledge receipt and process later, so the source service does not mark the delivery as failed and retry).