- An action or statement in response to a stimulus or other event
"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 decomposition
"In this reaction, the acid and base will neutralize each other, producing a salt."
"You were in the courtroom. What is your reaction?"
"The announcement of the verdict brought a violent reaction."