Wiki · Reference entry
Harness
The apparatus around a model that decides whether its output is allowed to matter.
A model on its own produces text. A harness is everything that turns that into something trustworthy or stops it: the checks, the scenarios, the required evidence, the gate that will refuse. Borrowed from testing, where a harness is the rig you run code inside rather than the code itself.
Why it matters
The model is the engine. The harness is the reason you are willing to drive it.