reactional Definition

1. A response to stimulus.
2. An action or change in the opposite direction.
3. A complete change of opinions, feelings, etc to the opposite of what they were.

Example: The idea was popular at first but then a reaction set in
Thesaurus: recoil, reversal, regression, backlash, rebound.
4. A response which shows how someone feels or thinks about something

Example: What was his reaction to the news?
Thesaurus: reply, response, answer, acknowledgment, rejoinder, feedback, retort, reciprocation.
5. Opposition to change, especially political change, reform, etc, and a tendency to revert to a former system, or state of affairs.
6. physiol.
A physical or mental effect caused by eg a drug, medicines, etc.
7a. chem.
A chemical process in which the electrons surrounding the nuclei in the atoms of one or more elements or compounds (reactants) react to form one or more new compounds or products (see product sense 4);
7b. chem.
Chemical change.
8. physics.
A nuclear reaction involving a change in an atomic nucleus, eg radioactive decay, nuclear fission, nuclear fusion.
9. physics.
The force offered by a body that is equal in magnitude but opposite in direction to the force applied to it.
10. stock exchange.
A sharp fall, in share prices, etc, after a rise.