- adjective referring to a chemical that reacts easily with other substances
- Tending to easily form part of a chemical reaction. For example, fluorine is the one of the most reactive of all chemical elements, and forms compounds with almost every other element, usually quite vigorously.
- Tending to respond to an action or stimulus. Also, tending to respond to an action or stimulus more than others within the same category or group.
- Pertaining to, causing, utilizing, or arising from reactance.
- adjective taking place as a reaction to something else
- adjective dealing with a problem once it has started to occur