ODI (oxygen desaturation index)
A number that shows how many times an hour the oxygen in your blood drops noticeably.
A little more detail
The number of drops in oxygen saturation of at least 3% (or 4%) from baseline per hour. It is calculated with pulse oximetry; on its own it is not considered enough to diagnose sleep apnea.
Source: Kapur VK, Auckley DH, Chowdhuri S et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(3):479-504. doi:10.5664/jcsm.6506