The term SONAR stands for Sound Navigation and Ranging. It is a technology that uses sound waves to navigate, communicate, or detect objects under or on the surface of the water. Sound waves are emitted and their echoes are received to determine the distance and characteristics of underwater objects. SONAR has two main types: active sonar, which emits sound pulses and listens for echoes, and passive sonar, which listens for sounds made by objects without emitting sound itself.