Definition of reactions:
(n) :
An action or statement in response to a stimulus or other event.
(n) :
(chemistry) A transformation in which one or more substances is converted into another by combination or decomposition.
(n) :
(Internet) An icon or emoji appended to a posted message by a user to express their feeling about it.
(n) :
(politics) Reactionary politics; a period in which reactionary thought or politics is resurgent or dominant.
(n) :
(Misesian praxeology, Austrian economics) Unpurposeful behavior.