The term 'acoustic REFLECTION' can be broken down into two words: 'ECHO' (4 letters) and 'REFLECTION'. ECHO is a sound wave that is reflected off of a surface, such as a wall or a mountain. REFLECTION is the process of bouncing back sound waves from a surface. Acoustic REFLECTION is the combination of these two processes, where sound waves are reflected off of a surface and then bounce back to the source. This phenomenon is used in many applications, such as sonar and medical imaging.