noun
secret, mystery
B1
Geheimnis: means 'secret' or 'mystery'. Neuter noun (das Geheimnis) with plural Geheimnisse. Regular declension but genitive often des Geheimnisses (added -es). Refers to hidden information or unexplained phenomena. Different from adjective 'geheim'. No special prepositions or reflexivity; common in everyday and literary use.
Examples
Das Geheimnis wurde endlich gelüftet.
The secret was finally revealed.
Niemand kennt das Geheimnis hinter dem alten Haus.
No one knows the mystery behind the old house.
Kannst du ein Geheimnis für dich behalten?
Can you keep a secret?
Details
Mnemonics
Picture a locked diary labeled 'Geheimnis'.
Sounds like 'guy-hime-niss' — imagine a guy keeping something tucked inside.
das — imagine a neutral box (das) holding the secret to remember neuter gender.
Notes
Can refer to a confidential fact or to something mysterious. Plural: Geheimnisse.