noun
advocate; ideologist; ideologue
Refers to a person who actively supports or promotes a particular theory, principle, or ideology. Often used in formal or intellectual contexts.
彼は自由主義の主義者として知られている。
He is known as an advocate of liberalism.
noun, used as a suffix
-ist (adherent)
Suffix attached to nouns denoting a belief system or principle to form a word for a follower or adherent. Very productive; e.g., 平和主義者 (pacifist), 菜食主義者 (vegetarian).
彼女は菜食主義者で、肉を一切食べない。
She is a vegetarian and doesn't eat any meat.
noun
socialist; communist; anarchist
Archaic use specifically referring to left-wing political adherents, especially socialists, communists, or anarchists. Now largely replaced by more specific terms like 社会主義者 or 共産主義者.
明治時代には、主義者という言葉が社会主義者を指すことがあった。
In the Meiji period, the word 主義者 sometimes referred to socialists.
社会主義者 specifically means 'socialist', whereas 主義者 is a broader term for any '-ist' or, archaically, a left-wing adherent.
Compound of 主義 (principle, doctrine) + 者 (person). The suffix usage is a direct calque from Western '-ist' formations.