a liquid that is applied to the surface of hardened concrete to either prevent or decrease the penetration of liquid or gaseous media, for example water, aggressive solutions, and carbon dioxide, during service exposer, that is absorbed by the concrete, is colorless, and leaves little or nothing visible on the surface. 

impervious to water in either liquid or vapor state.  (See also dampproofing.) (Because nothing can be completely “impervious" to water under infinite pressure over infinite time, this term should not be used.)

any sheet, plate, or layer of material attached directly to the inside face of formwork to improve or alter the surface texture and quality of the finished concrete.