Multilayer medium

In the physical sciences, a multilayer or stratified medium is a stack of different thin films. Typically, a multilayer is man made for a specific purpose. Since layers are thin with respect to some relevant length scale, interface effects are much more important than in bulk materials, giving rise to novel physical properties.
The term "multilayer" is "not" an extension of "monolayer" and "bilayer", which describe a "single" layer that is one or two molecules thick. A multilayer medium rather consists of several thin films.
Examples.
An optical coating, as used for instance in a dielectric mirror, is made of several layers that have different refractive indexes.
Giant magnetoresistance is a macroscopic quantum effect observed in alternating ferromagnetic and non-magnetic conductive layers.