an electrical device used to introduce reactance into a circuit
"The engineer installed a reactor in parallel with the motor to limit the starting current surge."
(physics) any of several kinds of apparatus that maintain and control a nuclear reaction for the production of energy or artificial elements
"The massive reactor at the power plant generates electricity by sustaining a controlled nuclear chain reaction."
A person who responds to a suggestion, stimulation or other influence.
"The reactor immediately expressed his doubts when the manager proposed raising prices again."
In plain English: A reactor is a machine that controls nuclear reactions to create energy for power plants.
"The nuclear reactor generates enough energy to power millions of homes."
Usage: Use this term specifically in psychology and social science contexts when describing someone's behavioral response to external stimuli rather than their internal thoughts. It is often confused with "respondent," but the former emphasizes the reaction itself while the latter refers broadly to anyone answering a survey or question.
The word reactor comes from combining the root react with the suffix -or. It originally meant one who reacts or responds to something.