Apnea-Hypopnea Index measures sleep apnea severity. The AHI is the sum of the number of apneas (pauses in breathing) plus the number of hypopneas (periods of shallow breathing) that occur, on average, each hour.
Working in collaboration with Dr. William C. Dement, Dr. Guilleminault established the apnea–hypopnea index, which is still in use today to characterize the presence and severity of sleep apnea.
To count in the index apneas and hypopneas, collectively called events, must have a duration of at least 10 seconds.
From the rating chart here, we see that an index less that 5 is considered normal. For an Apnea-Hypopnea Index from 5 to 15 denotes mild sleep apnea. Fifteen to 30 is moderate, while a greater than 30 is considered severe.