reflected wave; echo
Technical term used in physics, acoustics, and related fields for a wave that has bounced back from a surface or boundary.
Radar detects reflected waves to determine the position of objects.
In ultrasound examination, reflected waves from inside the body are turned into an image.
エコー is a common loanword for 'echo' in everyday contexts, whereas 反射波 is a more technical term.
Compound of 反射 (reflection) + 波 (wave). The exact historical derivation is uncertain, but the components are straightforward.