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Telephone tapping (or wire tapping/wiretapping in the US) is the monitoring of telephone and Internet conversations by a third party, often by covert means. The telephone tap or wire tap received its name because, historically, the monitoring connection was applied to the wires of the telephone line being monitored and drew off or tapped a small amount of the electrical signal carrying the conversation. Legalized wiretapping by police or other recognized governmental authority is otherwise known as lawful interception.
Passive wiretapping attempts only to observe the flow and gain knowledge of the information it contains. Active wiretapping attempts to alter the data or otherwise affect the flow of data.