Informant (noun)

1. A person who supplies information, esp. to the police or other authority.
2. A person who gives information about something or someone, typically secretly or against the wishes of the person concerned.

Origin:

Mid 16th century (in the sense "one who informs"): from latin informant- "informing", from the verb informare.

Examples:

1. The police arrested the informant who had tipped them off about the drug deal.
2. The informant provided information about the inner workings of the criminal organization.
3. The company's financial reports were leaked by an informant.
4. The informant gave the journalist inside information about the company's practices.
5. The informant's testimony helped to convict the defendant.