What do we call a reflected sound that can be distinguished from the original sound, typically arriving at least 0.1 seconds after it?

The term used to describe a reflected sound that can be distinguished from the original sound, typically arriving at least 0.1 seconds later, is "echo." An echo occurs when sound waves bounce off a surface—such as a wall or a cliff—returning to the listener with a delay that allows them to perceive it as a separate sound from the original. The stipulated time difference of 0.1 seconds is essential because it helps the human brain distinguish between the original sound and the reflection, creating the perception of two distinct sounds.

Reverberation, on the other hand, is the persistence of sound in an environment due to multiple reflections, but it does not create a clear distinction from the original sound. Instead, it blends with it, resulting in a prolonged sound rather than a separated one. Reflection refers to the phenomenon of sound bouncing off a surface but does not itself denote the time delay required for distinguishability. Resonance involves the amplification of sound due to matching frequencies, which is unrelated to the concept of distinct echoes. Thus, the definition and conditions met by an echo make it the correct choice in this context.