1 A psychological state resulting from the elimination or threatened elimination of an option, causing that option to appear more attractive. For example, if a child who initially has the possibility of playing with either of two toys is forbidden to play with one of them, then the forbidden toy may suddenly seem more attractive to the child. According to reactance theory, a person who is subjected to excessive persuasion that is experienced as a threat to freedom of choice may respond by strengthening the targeted attitude rather than changing it, and a person with a nagging spouse may deliberately avoid doing what the spouse continually demands. The concept was introduced in 1966 by the US psychologist Jack W(illiams) Brehm (1928–2009) in a book entitled A Theory of Psychological Reactance. See cognitive dissonance.
2 In engineering, opposition to the flow of alternating current, measured in ohms, resulting from the presence of an inductor or capacitor in an electrical circuit, or opposition to acoustic or mechanical vibration, usually resulting from inertia or stiffness of the vibrating object or medium. [From Latin reagere to react, from agere to do + -antia indicating an action, state, condition, or quality]