American Dictionary of the English Language

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IMPER'VIOUS, adjective [Latin 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.