To 'eavesdrop' is to surreptitiously overhear a private conversation.
History
Ancient
Anglo-Saxon law punished eavesdroppers, who skulked in the
Eavesdrip of another's home, with a fine.
Techniques

"Belly-buster" hand-
audio listening devices. After assembly, the base of the drill was held firmly against the stomach while the handle was cranked manually. This kit came with several drill bits and accessories.
Eavesdropping can also be done over
telephone lines (
wiretapping),
email,
instant messaging, and any other method of
communication considered private. (If a message is publicly broadcast, witnessing it does not count as eavesdroppi
In ancient China, it is said that to prevent eavesdropping when discussing important matters, soldiers would instead draw the
characters on hands or papers.
The Canadian heroine
Laura Secord is famous for having eavesdropped on the plans of the American army and delivering this information to the British.
Eavesdropping in fiction
Eavesdropping is something of a
clichéd
plot device in fiction, allowing the hero or villain to gain vital information by deliberately or accidentally overhearing a conversation. For instance, in "
Letting In the Jungle" by
Rudyard Kipling,
Mowgli overhears the hunter
Buldeo telling some men that Mowgli's adopted mother
Messua is about to be
executed, so Mowgli sets about rescuing her.
See also
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Computer surveillance
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ECHELON
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Espionage
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Magic (cryptography)
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Man-in-the-middle attack
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Katz v. United States (1967)
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NSA warrantless surveillance controversy (December 2005-2006)
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Opportunistic encryption
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Privacy
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Secure communication
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Surveillance
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Telephone tapping
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Fiber tapping
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Ultra
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Keystroke logging