Interface
The question is too vague for a definitive answer; depending on context, "surface", "interface", or even just "boundary" might be used.
The wave is called a "reflected wave" when it bounces back off the boundary between two different materials.
A wave that separates two different media is called a boundary or interface wave. Examples include reflection and refraction at the interface between two materials.
Reflection.
Light refracts when is crosses the boundary between two materials. That means that is the light is coming in at one angle, then it will change direction at the interface and have a different angle moving out from the other side. There is an old formula called Snell's Law that predicts the exit angle in terms of the properties of the material, i.e. in terms of the index of refraction. There is reflection as well. Some light bounces off of a boundary and some is transmitted.
The boundary between two materials is called an interface.
The question is too vague for a definitive answer; depending on context, "surface", "interface", or even just "boundary" might be used.
The wave is called a "reflected wave" when it bounces back off the boundary between two different materials.
The boundary between the troposphere and the stratosphere is called the tropopause.
Such a boundary is called a front.
The boundary of mantle and core are called Gutenberg Discontinuity
A boundary where the plates are moving apart is called a divergent boundary. It is also called a constructive boundary because new crust is made there.
A front
It is called a fault
A front
The boarder between two plates is called a boundary.
It is called seismic wave reflection. When a seismic wave encounters a boundary between different materials with contrasting density or rigidity, some of the wave energy is reflected back towards the source.