mole; whistle-blower; informant; betrayer
Refers to someone who secretly passes inside information to an opposing party, often in espionage, organizations, or criminal contexts.
警察は組織に内通者がいることを突き止めた。
The police discovered that there was a mole inside the organization.
彼は会社の不正を内部告発した
Rare, old-fashioned use for someone who commits adultery. Modern Japanese typically uses 不倫相手 or 浮気相手.
He is a whistle-blower who exposed the company's wrongdoing.
In old novels, the word 内通者 can refer to an adulterous partner.
Compound of 内通 (secret communication) + 者 (person). The exact historical derivation is uncertain.