noun
A person who advocates peace and opposes war or violence.
彼は熱心な平和論者で、戦争に反対している。
He is a passionate pacifist and opposes war.
平和主義者 is a more common and general term for a pacifist or peace advocate, while 平和論者 can imply a more theoretical or argumentative stance on peace.
Compound of 平和 (peace) + 論者 (advocate, theorist).