re·ac·tive / rēˈaktiv/ • adj. showing a response to a stimulus: pupils are reactive to light. ∎  acting in response to a situation rather than creating or controlling it: a proactive rather than a reactive approach. ∎  having a tendency to react chemically: nitrogen dioxide is a highly reactive gas. ∎  Physiol. showing an immune response to a specific antigen. ∎  (of a disease or illness) caused by a reaction to something: reactive arthritis reactive depression. ∎  Physics of or relating to reactance: a reactive load.

reactive (ri-ak-tiv) adj. (in psychiatry) describing mental illnesses that are precipitated by events in the psychological environment. r. hypoglycaemia see hypoglycaemia.