/ree'ak tiv"i tee/ , 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.
[ 1885-90; REACTIVE + -ITY ]