In passive sentences, the agent is often marked by 'ni'.
この動詞の動作主は人間に限られる。
The agent of this verb is limited to humans.
主語 is a grammatical subject, which may not always be the agent; 動作主 is a semantic role.
Compound of 動作 (action, movement) and 主 (master, main entity). A straightforward kanji compound for the linguistic concept of agent.