SoDelayQueueSensor(3) on Linux
The text 428c8e66ced4, as Debian 7 (Wheezy) shipped it.
The SoDelayQueueSensor class is the abstract base class for priority scheduled sensors. Delay queue sensors are invoked upon various events not related to time occurrences. See documentation of subclasses to see which types of events can be surveilled by the builtin sensor types.