noun
A person who opposes political or social progress and wants to return to a previous state. Often used in political or historical contexts.
彼は改革に反対する反動主義者だ。
He is a reactionary who opposes reform.
保守主義者 is a conservative who prefers gradual change or preservation of existing institutions, while 反動主義者 is a reactionary who actively seeks to reverse changes and return to an earlier order.
Compound of 反動 (reaction) + 主義 (principle, -ism) + 者 (person). The term is a calque from Western political vocabulary.