reaction

noun

1. An action or statement in response to a stimulus or other event.

   “The 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.”

2. 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.”

3. Reactionary politics; a period in which reactionary thought or politics is resurgent or dominant.

   “And as Europe advanced year by year more deeply into the low featureless plains of reaction, censorship and mediocrity and the pestilential swamps of poverty, unhappiness and oppression, the image of the liberating revolution became ever more luminous.”

4. (Austrian)Unpurposeful behavior.
5. (Internet)An icon or emoji appended to a posted message by a user to express their feeling about it.