SONAR: NOUN. Acronym. SOund NAvigation and Ranging. Originally SONAR was primarily use in an ACTIVE mode. This entails transmitting a very powerful tone through a hydrophone and listening for return echos. It is possible to determine the distance and bearing to any object that returns an echo. Later SONAR began to be used passively to listen. Most ships emply SONAR in the form of a FATHOMETER which is used to determine depth. SONAR is more widely recognized in the form of a fish finder that can be purchased inexpensively adn is used to help fisherman locate fish underwater in a manner similar to a fathometer. Some animals use SONAR including whales and dolphins, some fish, and even mammals (bats).