RSS Cloud is an extension to the RSS protocol that enables realtime (immediate) push notifications or distributed publish/subscribe communication for feeds. This is done using a <cloud> element where more information is given on how a subscriber should request updates. RSS Cloud is not limited to RSS feeds but can also be used with other feed formats such as Atom. On September 7th 2009 WordPress became the first large supporter of the protocol, enabling the <cloud> tag for over 7.5 million hosted blogs on WordPress.com [1].
Another protocol providing roughly the same is PubSubHubbub. Contrary to RSS Cloud, PubSubHubbub push notifications include content of the updated feed entries and do not unsubscribe after 25 hours.[2]