Reaction

Old French reaction, from Latin reactio, from the verb reago, from re- ("again") + ago ("to act"). More at English re-, action.

Full definition of reaction

(plural reactions)

An action or statement in response to a stimulus or other eventThe announcement of the verdict brought a violent reaction.You were in the courtroom. What is your reaction?When I last tried to eat strawberries I had a terrible allergic reaction.
(chemistry) A transformation in which one or more substances is converted into another by combination or decompositionIn this reaction, the acid and base will neutralize each other, producing a salt.