Eavesdropping is the act of secretly listening to the private conversation of others without their consent, as defined by Black's Law Dictionary. This is commonly thought to be unethical and there is an old adage that "eavesdroppers seldom hear anything good of themselves...eavesdroppers always try to listen to matters that concern them."
Anglo-Saxon law punished eavesdroppers, who skulked in the eavesdrip of another's home, with a fine; the eavesdropper was also sometimes called the eavesdrop. Eavesdrop also means a small low visibility hole near the entrance to a building (generally under the eaves) which would allow the occupants to listen in on the conversation of people awaiting admission to the house. Typically this would allow the occupant to be prepared for unfriendly visitors.
Early telephone systems shared party lines which would allow the sharing subscribers to listen to each other's conversations. This was a common practice in rural America which resulted in many incidents and feuds.
Eavesdropping can also be done over telephone lines (wiretapping), email, instant messaging, and other methods of communication considered private. (If a message is publicly broadcast, witnessing it is not considered eavesdropping.) VoIP communications software is also vulnerable to electronic eavesdropping via malware infections such as trojans.
The verb eavesdrop was originally a back-formation of the noun eavesdropper ("a person who eavesdrops") which was formed from the unrelated noun eavesdrop ("the dripping of water from the eaves of a house; the ground on which such water falls"). An eavesdropper was one who stood at the eavesdrop (where the water fell, i.e., near the house) so as to overhear what was said inside.
See also 
- Computer surveillance
- Fiber tapping
- Katz v. United States (1967)
- Keystroke logging
- Magic (cryptography)
- Man-in-the-middle attack
- NSA warrantless surveillance controversy (December 2005-2006)
- Opportunistic encryption
- Secure communication
- Telephone tapping
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