It's anxiety that happens all the time and not comes and goes like it would under normal conditions. When you are OK and something stressful happens, you can get anxious but this anxiety will disappear once the cause of it ends (as in when you have to take an exam, for example - you are all anxious and nervous before and even while you are taking it but, once it's over, you are fine). When animals live under conditions that are inadequate and the conditions never get better, they end up with chronic anxiety. Animals in zoos (even good ones!) have it and develop the stereotypies (like tigers that pace their cage all day long, day after day after day; or elephants that swing their body going from one foot to another all the time; or dolphins that swim in a circle; parrots that pluck and self mutilate, etc). Basically, when an animal is forced to live a life completely different than what nature evolved them for and this life brings on causes for anxiety all the time, the consequence is chronic anxiety.