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main constituent; core; nucleus
subject
「主体」 has another reading with different meanings.
このプロジェクトは学生が主体となって進めている。
This project is being led by students as the main participants.
地域住民が主体となってイベントを企画した。
Local residents took the lead in planning the event.
自分の人生は自分が主体的に決めるべきだ。
You should be the one to proactively decide your own life.