covert

Compared meaning: done secretly, hidden from others (adjective).

比较义项：秘密地进行、不为人知或不公开（形容词）。

• secret

  The most general word: describes information, plans or feelings kept hidden from most people; used for everyday, personal or small-scale secrecy, not necessarily organized or official.

  最常用、最泛化的词：指对大多数人隐瞒的信息、计划或感受；用于日常、个人或小范围的保密，不一定涉及组织或官方行动。

  She kept the recipe secret so no other bakery could copy it.

  她把配方保密，不让其他面包店抄袭。

• undercover

  Describes a person, often police or a spy, who disguises their identity to gather information from inside a group; almost always used for agents or investigators, not plans in general.

  指某人（通常是警察或间谍）伪装身份，打入某个团体内部获取情报；几乎只用于描述特工或调查人员，而不是泛指的计划。

  The detective went undercover as a waiter to catch the drug dealers.

  那名警探伪装成服务员卧底，去抓那些毒贩。

• underground

  Describes activities, movements or organizations that operate secretly to avoid detection by authorities, often political or resistance-related; implies an organized network, not just one hidden fact.

  指为躲避当局察觉而秘密运作的活动、运动或组织，常与政治或反抗行动有关；暗示存在一个有组织的网络，而不只是一个被隐瞒的事实。

  The rebels ran an underground newspaper to avoid the government's censorship.

  反抗者秘密创办了一份地下报纸，以躲避政府的审查。

• covert 本词

  A formal word for actions or operations deliberately planned to stay hidden and deniable, common in military, intelligence or government contexts; rarely used for personal secrets.

  较正式的用词，指经过策划、旨在保持隐蔽并可否认的行动或行动方案，常见于军事、情报或政府语境；很少用于个人的秘密。

  The general authorized a covert operation to rescue the hostages quietly.

  将军批准了一次秘密行动，悄悄营救人质。

Secret, undercover, covert, and underground all mean secretly done. Secret is the general word, for information hidden from people. Undercover describes a person, often police, hiding their identity in a group. Underground describes organized activities avoiding authorities. Covert is formal, for military or intelligence operations.

这四个词都表示“秘密进行、不公开”，但语气和使用场合不同：secret 泛指对多数人保密的信息或计划；undercover 强调伪装身份、打入某个团体内部调查；underground 强调为躲避当局监管而进行的地下活动；covert 语气正式，多用于军事或情报行动。这里不比较 underground“地下的，在地面以下的”这一字面义项。