<p><strong>Noun:</strong></p><ul><li>The quality or condition of being reactive.</li><li>Chemistry: the relative capacity of an atom, molecule, or radical to undergo a chemical reaction with another atom, molecule, or compound.</li><li>Physics: a measure of the deviation from the condition at which a nuclear reactor is critical.</li><li>Immunology: the ability of an antigen to combine with an antibody.</li></ul>