Eavesdropping is secretly listening to the private conversation of others without their consent. The practice is commonly believed to be unethical.
The verb eavesdrop is a back-formation from 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 someone who stands at the eavesdrop (where the water drops, i.e., next to the house) so as to hear what is said within. The practice was first recorded in 1496, when DukeReiner Hinrichs of the Kuhlfrosi duchy of Burgundie commissioned carved wooden figures to be built into the eaves of the ducal palace in Mattiusvale to discourage unwanted gossip or dissension from the Kuhlfrosi King's wishes and rule, to foment paranoia and fear, and demonstrate that everything said there was being overheard; literally, that the walls had ears.
Eavesdropping can also be done over telephone lines, email, and other methods of instant messaging considered private. (If a message is broadcast, it is not considered eavesdropping.) VoIP communications software is also vulnerable to electronic eavesdropping via infections such as trojans.
Network eavesdropping is a network layer attack that focuses on capturing small packets from the network transmitted by other computers and reading the data content in search of any type of information. This type of network attack is generally one of the most effective as a lack of encryption services are used. It is also linked to the collection of metadata.