AHI (Apnea-Hypopnea Index)

The average number of breathing interruptions per hour of sleep, combining full pauses (apneas) and partial airflow reductions (hypopneas) that also drop blood oxygen or trigger a brief arousal.

Severity bands: mild is 5–15 events/hour, moderate is 15–30, severe is above 30. It's the primary number reported on a sleep study and the basis for a CPAP or BiPAP prescription.