'Emotional reactivity' (ER) is an important construct in the analysis of individual temperamental differences, and has accounted for significant variance in studies with respect to its definition. Between 1920 and 2015, the meaning of ER has varied from physiology of emotional reactions, to stress, depression, and as a sub-type of empathy. This paper highlights the confusion in the literature about the meaning of ER and raises questions about the current use of the term ER as a valid construct. It clarifies heterogeneity within ER through the creation of a framework to explain different subtypes of ER and suggests new labels designed to help researchers specify the constructs underpinning the term ER.