* Sound is the fundamental element. Sonar relies on the transmission and reception of sound waves to determine distances, shapes, and locations of objects underwater.
* Echo is the crucial phenomenon. Sonar works by emitting sound waves and then analyzing the echoes that bounce back from objects.
* Reflection is the mechanism by which echoes are created. Sound waves, when they encounter a boundary between two different mediums (like water and a fish), are reflected back.
Therefore, the core words that define sonar are sound, echo, and reflection.