A medium describes volumic material properties. Currently, this includes refractive index and light absorption properties, generalized to complex refractive indices or equivalently permittivity (also called dielectric function)
Media also have a precedence parameter, which controls the medium filling a volume in the scene when multiple geometries with different media are overlapping.
This medium allows defining any dielectric function for describing homogenous isotropic materials ranging from dielectrics (water, glass, ...) to semiconductors and metals.
This special medium is not a medium by itself, but a link to another medium defined in the scene. It allows referencing the same medium multiple times or using a medium from a library.