IMPER'VIOUS, a. [L. impervius; in and pervius, passable; per and via, way.]1. Not to be penetrated or passed through; impenetrable; as an impervious gulf; an impervious forest.2. Not penetrable; not to be pierced by a pointed instrument; as an impervious shield.3. Not penetrable by light; not permeable to fluids. Glass is pervious to light, but impervious to water. Paper is impervious to light. In the latter sense only, impervious is synonymous with impermeable.