Unremovable Ko Threat
An unremovable ko threat is a ko threat which cannot be removed without also executing the threat.
Unremovable ko threats typically occur in seki positions; here is an example. White has a ko threat here at a, which Black can never remove. Removing the ko-threat would mean Black playing at a himself, which of course leads to instant capture.
Sometimes dead groups contain unremovable ko threats for unremovable kos.