noun
top secret matter; closely-guarded secret
Refers to a matter or piece of information that is kept strictly confidential, often in official or organizational contexts.
その文書は極秘事項として扱われている。
The document is treated as a top secret matter.
極秘事項が外部に漏れた。
A closely-guarded secret was leaked to the outside.
Compound of 極 (ごく, 'extreme') + 秘 (ひ, 'secret') + 事項 (じこう, 'matter'). The combination straightforwardly means 'extremely secret matter'.