Desaturation
A short drop in the level of oxygen in the blood. When breathing weakens, the blood's oxygen store empties a little.
A little more detail
A drop in oxygen saturation of at least 3% (4% under some rules) from baseline. It is used in scoring hypopneas and in calculating the ODI.
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