Stress: originally considered as a displaced homeostasis leading to new adaptations (Stott, 1981), stress is now linked to the perceived situation of the animal, i.e. the emotional state resulting from a situation that is judged to be inappropriate, incompatible with expectations, threatening, etc. (from Veissier & Boissy, 2007). 

Veissier, I., & Boissy, A. (2007). Stress and welfare: Two complementary concepts that are intrinsically related to the animal’s point of view. Physiology & Behavior, 92(3), 429-433. 10.1016/j.physbeh.2006.11.008