Pronunciation: (rē"ak-tiv'i-tē),[key] —n. 1. the quality or condition of being reactive.
2. Chem.the relative capacity of an atom, molecule, or radical to undergo a chemical reaction with another atom, molecule, or compound.
3. Physics.a measure of the deviation from the condition at which a nuclear reactor is critical.
4. Immunol.the ability of an antigen to combine with an antibody.