self-hosted runner
A CI service's runner program installed on a computer you own.
A self-hosted runner is the CI provider's agent running on the user's own hardware; it runs that user's jobs and is how a rig with attached boards joins CI.
Examples
The Raspberry Pi from Chapter 16 can be registered as a self-hosted runner, so CI jobs run on a machine you own.
A hardware-in-the-loop rig joins CI as a self-hosted runner, because the boards are plugged into it.