The quality or condition of being reactive, or the degree to which something is reactive. The reactivity of the chemical compound was tested in the laboratory.
The tendency of a substance to undergo chemical reaction, either by itself or with other materials, and to release energy. The reactivity of sodium with water is well-known and can be quite explosive.
The responsiveness of a person or system to a stimulus or change in conditions. The reactivity of the stock market to political news can be unpredictable.
In nuclear engineering, a dimensionless measure of the departure of a nuclear reactor from criticality. The control rods were adjusted to decrease the reactor's reactivity and maintain safe operation.