Hypopnea
When breathing becomes clearly weaker during sleep without stopping. Like a tap turned down rather than off.
A little more detail
Under the AASM 2012 recommended rule, a reduction in flow of at least 30% in the nasal pressure signal, lasting at least 10 seconds; it must be accompanied by an oxygen desaturation of at least 3% or an arousal. Some payers, such as Medicare, use a 4% desaturation requirement.
Source: Berry RB, Budhiraja R, Gottlieb DJ et al. Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual for the Scoring of Sleep and Associated Events. J Clin Sleep Med. 2012;8(5):597-619. doi:10.5664/jcsm.2172