Separate emotional states… (which) can be identified and differ not only in expression but in other important ways such as appraisal, antecedent events, behavioural responses and physiology. Discrete emotions also have to be in some way adaptive.
Ekman, P. (1992). An argument for basic emotions. Cognition & Emotion, 6(3-4), 169-200. doi: 10.1080/02699939208411068; Ekman, P., & Cordaro, D. (2011). What is meant by calling emotions basic. Emotion Review, 3(4), 364-370. doi: 10.1177/1754073911410740