We addressed this question to a clinical psychologist we know (R.H.):

How is reactivity commonly used?

His reply:

Basically, a stimulus occurs (internal or external) and based upon the reactivity (the combination of innate sensitivity and personal history of the person) he will immediately search (inwardly... but usually outwardly) to locate a plausable explanation for the emotion/sensation that immediately follows the stimuli. This is called "Attribution Theory" in which a (usually simple) explanation is rapidly arrived at to explain something unexpected. Of course the "anal retentives" will not settle on a simple answer at all, and, avoiding the law of parsimony (Ocam's Razor), will attribute varied and exotic explanations for the stimuli. Reactivity is the tendency (frequency, duration, and intensity) to respond in an irrational manner. Prozac, Thorazine, Valium and Lithium Carbonate help to reduce reactivity... but then again, so does a POST-frontal lobotomy.