interlayer

This term is primarily used in technical, engineering, and materials science contexts. It describes a functional boundary or a physical medium that mediates the relationship between two primary strata. While it can refer to a physical substance like a glue or a membrane, it also describes the spatial relationship between those strata.

In professional discourse, it is often contrasted with the outer layers or the substrate. When used as an adjective, it specifies the location of a property or a force, such as tension or friction, that exists exclusively within that middle zone.