noun
A person who advocates peace and opposes war or violence as a matter of principle.
彼は熱心な平和主義者で、戦争には絶対反対だ。
He is a devoted pacifist and is absolutely opposed to war.
平和主義 is the abstract principle of pacifism, while 平和主義者 refers to a person who holds that principle.
Compound of 平和 (peace), 主義 (principle, -ism), and 者 (person).