What do we call the border between two different media?

The term that refers to the border between two different media is "interface." In physics and various scientific fields, the interface is where two distinct physical substances or environments meet, such as the surface of water separating air from liquid or the transition between two different materials in optics and acoustics.

Other terms, like "boundary," while often used interchangeably in casual contexts, typically refer to abstract limits or edges of a space rather than the specific meeting point of two different physical states. "Conduit" relates to a channel for conveying substances rather than describing a junction. "Medium" refers to the material through which energy or waves travel but does not specifically denote the interface between two different materials. The clarity of these definitions helps ensure a thorough understanding of the concepts of media and interactions at interfaces in physical science.