Explanation:

- A person who listens to someone’s private conversation without them knowing typically does so without consent.

- Eavesdropper: This is the correct term. It means someone who secretly listens to the conversations of others.

- Spy: A spy gathers information secretly, usually about another country or organization. They might eavesdrop, but spying is broader.

- Secret agent: Similar to a spy, they are involved in undercover work and might listen in on conversations, but they have a larger purpose.

- Infiltrator: This person secretly gains access to a place or organization. Listening in might be part of their role, but it isn’t their defining feature.

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