1.
main body, core, principal part
The central or primary component of something. Used to identify the main actor, agent, or entity responsible for an action or project.
Usage
The adjective form 主体的 (proactive, self-directed) is very commonly used in education and business contexts, meaning to act on one's own initiative rather than passively following instructions.
The pattern ~主体 is used as a suffix meaning "-led" or "-centered" (e.g., 学生主体 = student-led).