Echo, a repeated sound. The term is usually used to refer to sound that is reflected from a surface and returned to its source after a noticeable delay. A person can typically hear his or her own echo where a hard surface, such as the side of cliff, reflects sound waves directly back to the person. The use of echoes to locate objects is called echolocation.
American Meteorological Society, a scientific, educational, and professional society of meteorologists and others interested in the atmospheric sciences.
Nebulae are collections of dust and gases scattered across the galaxy. They're the sites where stars are born and what's left behind after they die.