A Babel fish is a highly improbable biological universal translator. It appears as a "small, yellow and leechlike" fish. When a Babel fish is inserted into the ear canal it allows the 'wearer' to "instantly understand anything said... in any form of language." This was both a useful plot device for Adams, who wrote on the subject that he always found the ability of all aliens to speak English very strange; and also the starting point for a joke about the existence of God.
According to the Hitchhiker's Guide, the Babel fish was put forth as an example for the non-existence of God:
The Babel fish inspired the name of the Babel Fish online text translation service provided by the web portal AltaVista. It should be noted, however, that the existence of this online service "in no way proves or disproves God's existence on the Internet."
The Babel fish also inspired the name of the Babel Fish Corporation, a translation service which is not affiliated with Alta Vista's Babel Fish translation service.
See also: Tower of Babel, fishbait