REI (respiratory event index)
The number of breathing events per hour counted in a sleep test done at home. Because it is not known whether you were asleep, it is divided by the total recording time.
A little more detail
Found by dividing the number of apneas and hypopneas in a home sleep apnea test by the monitoring time instead of the sleep time. Because time spent awake enters the denominator, the REI can understate the AHI of the same night.
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