also: ないしょうごと
noun
secret; private matter; confidential matter
Refers to something kept secret or private, often among a small group. Commonly used in phrases like 内緒事にする (to keep something secret).
これは二人だけの内緒事だよ。
This is a secret just between the two of us.
会社の内緒事を外部に漏らしてはいけない。
You must not leak the company's confidential matters to outsiders.
Compound of 内緒 (ないしょ, 'secrecy') and 事 (こと, 'matter'). The kanji 内証 is an alternative spelling for 内緒, but 内証事 is rare.