Some species of moths have "developed a unique defense" characteristic of sounding like a bat and therefore fooling a bat's sense of echolocation. What is this defense characteristic an example of?

The answer is anti-predator adaptation. This occurs when prey establishes defense mechanisms to promote its survivability in an environment it shares with its predators / enemies. The first line of defense consists in avoiding detection. Alternatively, prey animals may ward off an attack by the predator.