Trass \Trass\, n. [D. tras or Gr. trass, probably fr. It. terrazzo terrace. See {Terrace}.] (Geol.) A white to gray volcanic tufa, formed of decomposed trachytic cinders; -- sometimes used as a cement. Hence, a coarse sort of plaster or mortar, durable in water, and used to line cisterns and other reservoirs of water. [Formerly written also {tarras}, {tarrace}, {terras}.] [1913 Webster] Note: The Dutch trass is made by burning and grinding a soft grayish rock found on the lower Rhine. [1913 Webster]