"How would you define an "emotion"?

There has been ongoing debate for decades about what "emotion" means, and there is no generally accepted definition. In an article that Ralph Adolphs [Bren Professor of Psychology and Neuroscience and Professor of Biology] and I recently wrote, we put forth the view that emotions are a type of internal brain state with certain general properties that can exist independently of subjective, conscious experience. That means we can study such brain states in animal models like flies or mice without worrying about whether they are consciously aware or not. We use the behaviors that express those states as a readout. For example, behaviors that express the emotion state we call "fear" are freezing and flight. Behaviors that express "anger" include various forms of aggression."