Communication between browser and WordPress per AJAX in fixed intervals.
The Heartbeat API exists since WordPress 3.6, see Trac ticket #23216 (with an attached test plugin).
- WordPress Pulse? – Heartbeat API by @bueltge, German translation
- The Heartbeat API: Getting Started by Stephen Harris
- Using the WordPress Heartbeat API by Pippin
- wp-heartbeat-notify by Alessandro Benoit – Plugin using the API to send real time messages to visitors