An Etymological Dictionary of Astronomy and Astrophysics

1. General: The quality or condition of being reactive.
   

2. Physics: A measure of the deviation from the condition at which a → nuclear reactor is critical.
   

3. Chem.: The relative → capacity of an → atom, → molecule, or → redical to undergo a → chemical reaction with another atom, molecule, or → compound.

See also: From → reactive + → -ity.