Onboard (Companion) Computer
Onboard Computers are usually small single board computers (SBCs) which can be connected to an Autopilot Board, and can facilitate MAVLink communications with the Control Station as well as enable interfacing and processing of sensors the autopilot is poorly suited to handle.
When paired with an independent Autopilot Board (e.g. Pixhawk) the Onboard Computer is sometimes called a Companion Computer. Other boards like the Navigator use the Onboard Computer as a host, and cannot run without one.
An ArduSub-based vehicle is expected to include an Onboard Computer running AirOS or the older ArduPlane, ArduRover, ArduSub Companion Computer Software (often refered to as the "Companion Software").
Supported Hardware
AirOS can run on Raspberry Pi boards from the 3 Model B and onwards. It is designed to be portable, and support for additional Linux-based SBCs will come in future. Contributions are welcomed.
The Companion Software is only supported for use with the Raspberry Pi 3 Model B.
There is a comparison of Onboard Computer specifications in the Airdroper Technical Reference.
References
The ArduPilot documentation has more reading material about Companion Computers, but those software images are not compatible with the ArduPlane, ArduRover, ArduSub system.