PSM (Power Saving Mode)
Updated
PSM is a 3GPP feature that lets a device stay registered on the network while it sleeps for long periods, unreachable, then wake up and send data without registering again. The device and the network agree on timers that set how long it may sleep and how long it stays reachable after each transmission.
In practice
PSM can stretch the battery life of a sensor to years, but the network must support it and the device cannot receive commands while asleep.