
der Deckname
code name
An assumed, false name used to hide the true identity of a person, organization, or operation.
Der Spion operierte unter dem Decknamen „Falke“.
The spy operated under the code name 'Falcon'.
code name
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code name
An assumed, false name used to hide the true identity of a person, organization, or operation.
Der Spion operierte unter dem Decknamen „Falke“.
The spy operated under the code name 'Falcon'.
The word is made up of 'decken' (in the sense of to hide/cover) and 'Name' (name). So a 'Deckname' (code name) is literally a name that 'covers' or hides a person's true identity.
Die Autorin veröffentlichte ihre Bücher unter einem Decknamen, um privat zu bleiben.
The author published her books under a code name in order to remain private.
'Deckname' (code name) is almost always used with the preposition 'unter' (under). The fixed expression is 'unter einem Decknamen' (under a code name), followed by an activity.
Der Agent reiste unter einem Decknamen nach Moskau.
The agent traveled to Moscow under a code name.
Common verbs used with 'Deckname' (code name) are 'benutzen' (to use), 'verwenden' (to use), 'tragen' (to bear/carry), or 'operieren' (to operate).
Er musste für die Mission einen neuen Decknamen verwenden.
He had to use a new code name for the mission.
alias
Alias generally refers to an alternative name that a person uses in addition to their real name. A Deckname, by contrast, is an assumed false name whose specific purpose is to hide the person's true identity. Without this purpose, Alias is the appropriate word; when deliberate secrecy is involved, Deckname is more suitable.

Lea nutzt „Morgenrot“ während der geheimen Operation als Decknamen, um ihre Identität zu verbergen.
Lea uses “Morning Red” as a code name during the secret operation to hide her identity.
Lea nutzt „Morgenrot“ im Forum als Alias neben ihrem echten Namen.
Lea uses “Morning Red” as an alias on the forum alongside her real name.