(Hearing Access Profile)
A Bluetooth profile that enables a device (like a phone) to connect directly to one or two hearing aids via LE Audio, the newer standard for all kinds of audio over Bluetooth.
See: LE Audio
While HAP allow hearing aids to function like regular Bluetooth earbuds, it also includes several features specific to hearing aids. For example, it allows separate volume controls for ambient sound, telecoil audio, and audio streams from the phone or a broadcast source. The user might also be able to control hearing aid presets from their phone.
HAP includes an "Immediate Alert" function for a device like a phone to capture the attention of the hearing aid wearer, so the wearer doesn't miss important events like an incoming phone call or doorbell ring.
While Bluetooth 5.2 (and newer) and lower-level profiles such as BAP and CAP lay a foundation for LE Audio, it is the higher-level "Use Case" Profiles such as HAP that actually implement the features you use.
Other Use Case profiles include TMAP (Telephony and Media Audio Profile) and PBP (Public Broadcast Profile).