(Science: geology) 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.
Origin: D. Tras or Gr. Trass, probably fr. It. Terrazzo terrace. See Terrace.