One who listens to the conversation of others without their knowledge?

A person who listens to the conversation of others without their knowledge is often referred to as a "eavesdropper." This behavior is typically considered intrusive and unethical, as it violates the privacy of the individuals involved in the conversation. Eavesdropping can occur in various settings, whether in public spaces or through electronic means, and can lead to misunderstandings or breaches of trust.