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.