Webhook notifications (sometimes referred to as "push notifications") are a type of API in which a server on the client end (call-back server) can subscribe to the host server and messages or response data from the host server are sent to the client via a URL the client has provided to the host. Traditionally, REST APIs call or "pull" data from the server by calling an endpoint and waiting to receive the data. The caller must do this each time they want new data which can be an expensive and inefficient operation if this data is required to be polled frequently. Webhook notifications provide a more efficient solution by "pushing" this data to a URL the client has provided which listens for new data from that endpoint.