Arousal
When the brain slips out of sleep for a few seconds and goes back. You do not remember it in the morning, but if it happens often, sleep is not restful.
A little more detail
Under the AASM rule, an abrupt shift in EEG frequency lasting at least 3 seconds after at least 10 seconds of stable sleep; in REM an increase in chin EMG is also required. It plays a part in ending respiratory events and in scoring hypopneas.
Source: Bonnet MH, Doghramji K, Roehrs T et al. The scoring of arousal in sleep: reliability, validity, and alternatives. J Clin Sleep Med. 2007;3(2):133-145. doi:10.5664/jcsm.26815