The board made important decisions during a closed-door session.

closed-door 是由形容词 closed（关闭的）和名词 door（门）组成的复合形容词，字面意思是"门关着的"，引申为"不公开的、秘密的"。 closed-door session 指"闭门会议"，即不对外公开、只有特定人员参加的会议。