transparent; plain (glass or glasses)
Describes glass or lenses that are clear, without tint, pattern, or prescription. Often used for plain eyeglasses or window glass.
This window has plain glass, so you can see outside clearly.
He wears plain glasses (non-prescription).
Compound of 素 (plain, unadorned) and 通し (passing through), literally 'plain pass-through'. The exact historical derivation is uncertain, but the meaning follows naturally from the components.