What is the name of the wave that strikes a boundary between two media?

The correct answer is the incident wave. This term specifically refers to the wave that approaches and strikes the boundary between two different media. When a wave travels from one medium to another, it reaches a point where it interacts with the boundary, and it is at this moment that it is termed the incident wave.

Understanding the role of the incident wave is crucial in wave physics. Upon reaching the boundary, several phenomena can occur: part of the wave may be reflected back into the original medium, part may be transmitted into the second medium, and part may be refracted depending on the properties of the media involved. The incident wave sets the stage for these interactions to occur, making it a foundational concept in the study of wave behavior. This is why identifying and correctly naming it is important.