Fight-or-flight response refers to an organism's response when faced with an acute threat to survival. It is marked with physical changes that include nervous and endocrine changes preparing the organism to react or to retreat.
For example, when a snake faces a predator or a threat, its responses could be crawling away or acting as if it is ready to attack as its defensive displays. It could be through hooding, hissing, and raising the upper portion of their bodies to stand erect.