The patient insisted he saw people in the room who weren't there, and the doctor suspected he was hallucinating.

hallucinating 是动词 hallucinate 的现在分词形式。hallucinate 由词根 hallucin-（来自拉丁语，表示"幻觉"）和动词后缀 -ate 组成，意思是"产生幻觉"。hallucinating 在这里表示"正在经历幻觉"的状态。