Eavesdropping is the intercepting and reading of messages and conversations by unintended recipients. One who participates in eavesdropping, i.e. someone who secretly listens in on the conversations of others, is called an eavesdropper. The origin of the term is literal, from people who would literally hide out in the eaves of houses to listen in on other people's private conversations.
Eavesdropping can also be done over telephone lines, email, instant messaging, and any other method of communication considered private. (If a message is publicly broadcast, witnessing it does not count as eavesdropping.)
Messages can be protected against eavesdropping by employing a security service of confidentiality (or privacy). This security service is usually implemented by encryption.
In ancient China, it is said that to act against 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 the plans of American army and delivered this information to the British.
"Belly-buster" hand-crank audio drill kit
The picture at right shows a "belly-buster" hand-crank audio drill kit, which was used during the late 1950s/early 1960s to drill holes into masonry for implanting 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.