iAP or iPod Accessory Protocol is an Apple-designed software/firmware protocol used for accessories to connect to Apple iOS devices. The iAP is used for communication between the firmware and applications and is used as an intermediary for almost all accessories.
…the iPod Accessory protocol specification, the communication protocol used to interact with iPod, iPhone, and iPad.MFi Program – Apple Developer Programs
Originally introduced with the specific purpose of setting a protocol for in-line music controls with the 3.5mm headphone jack on the iPod, it has now grown to cover all iOS devices using all kinds of connectors such as the now-discontinued 30 pin connector, the Lightning connector, and 3.5mm headphone jack and even Bluetooth connections.
Not all developers can access the iAP protocol. Access to the protocol specification comes with MFi Certification Program.
iAP 1: Original iteration of the iPod Accessory Protocol.
iAP 2: Released with the announcement of the iPhone 5 and the subsequent release of the Lightning connector. With the release of iAP 2, support for both Lightning and Bluetooth was added.
Although some refer to iAP as the Apple interface accessory protocol, Apple’s official MFi Certification Program page refers to the iAP standard as the iPod Accessory Program.