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.
The document is treated as a top secret matter.
A closely-guarded secret was leaked to the outside.
極秘 is a noun or no-adjective meaning 'top secret' or 'strictly confidential', often used attributively. 極秘事項 specifically refers to a secret matter or item.
機密事項 also means 'confidential matter', but 極秘事項 implies an even higher level of secrecy, with 極 emphasizing 'extreme' or 'utmost'.
Compound of 極 (ごく, 'extreme') + 秘 (ひ, 'secret') + 事項 (じこう, 'matter'). The combination straightforwardly means 'extremely secret matter'.