adj

• (politics) Extremely conservative; opposing revolution (such as the French Revolution); favoring a return to a golden age of the past.
  "There's a fairly simple reason for the embrace of radicalism on the right, and it has to do with the reactionary imperative that lies at the core of conservative doctrine. […] If he is to preserve what he values, the conservative must declare war against the culture as it is."
• (chemistry) Of, pertaining to, participating in, or inducing a chemical reaction.
  "Psychiatry extends the theory into biology in the belief that all human behavior is nothing more than a series of reactionary chemical processes in the brain that determine pre-coded genetic responses built up from the conditioning of one’s environment."

noun

• (politics) One who opposes revolution, wanting to reverse it, favoring a return to a past golden age. Often used as a derogatory by revolutionaires.
  "Hindu reactionaries, whose conception of a well-ordered society had not moved beyond the laws of Manu, fell into line for the moment with the intellectual products of the modern Indian University."