The reflective metal layer and protective coating system applied to the rear surface of a conventional silvered flat glass mirror.
GlassRenu · Educational illustration
Meaning and boundaries
The backing is not ordinary exposed glass and may include multiple manufacturer-specific layers; it is distinct from the front glass surface and from first-surface mirrors.
Why it matters in glass restoration
Abrasion, solvent, liquid, adhesive, heat, and edge contact can permanently damage reflectance or accelerate corrosion.