I'm looking for a technical (medical or biological) term for when an animal freezes in response to a perceived danger. This is a pretty well known phenomenon (https://en.wikipedia.org/wiki/Fight-or-flight_response), i.e. a deer freezing in the headlights of a car, but I'm having trouble determining if there's a more precise word/phrase.
It is not actually "fight-or-flight" but rather tonic immobility. It is an instinctive reaction in many prey animals which is similar to "playing possum" or feigning death. It increases the chance that an animal's natural camouflage will work to hide it from a predator. It doesn't deter cars, though.