Plain English Meaning
An EFB that is built into the aircraft as permanent equipment, approved by the FAA the same way other aircraft instruments are approved, instead of being a tablet the pilot brings on board.
Full Definition
An Electronic Flight Bag that is permanently mounted in the aircraft and integrated into its systems, requiring FAA airworthiness approval through the certification or supplemental type certificate (STC) process. Because it is part of the aircraft, an installed EFB is treated as aircraft equipment rather than a portable device and is governed by 14 CFR Parts 21, 23, 25, 27, and 29 rather than by the operational EFB guidance that applies to portable units.
Context Anchor
Seen in EFB guidance when deciding whether the device is portable equipment or part of the aircraft installation.