secret; clandestine; confidential:
unravel the secrets of the universe;
揭開宇宙的秘密
The senate had a session today with closed doors.
參議院今天召開壹次秘密會議。
Do it, by all means, but strictly on the quiet.
這件事壹定要幹， 不過要嚴守秘密。
短語:
秘密報告 secret report;
秘密串連 make secret contacts with sb.;
秘密電報 {訊} bamboo telegraph; bamboo wireless;
秘密鬥爭 underground struggle;
秘密會議 secret meeting; closed-door session;
秘密活動 clandestine activties;
秘密計劃 clandestine plan;
秘密交易 clandestine dealing;
秘密接頭 {訊} wiretap;
秘密結社 form a secret society;
秘密警察 secret police;
秘密貿易 clandestine trade;
秘密內閣 interior cabinet;
秘密渠道 back channel;
秘密條約 secret treaty;
秘密通信 confidential correspondence;
秘密投票 secret ballot; Australian ballot;
秘密外交 secret diplomacy;
秘密衛星 unannounced satellite;
秘密文件 secret papers; confidential document;
秘密文件指示符 backdoor file indicator;
秘密武器 {軍} [德] wuwa; secret weapon;
秘密消息 classified information;
秘密協議 knockout agreement;
秘密選舉 secret election;
秘密準備 undisclosed reserve;
秘密資產 nonledger assets
參考：有道詞典