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.]
⇒ The Dutch trass is made by burning and grinding a soft grayish rock found on the lower Rhine.
© Webster 1913.