n. n. In mech.: A name given to rounded formations suggesting the carapace of the tortoise or turtle.n. A rounded-back cover of any structure, as over the rear of a motorcar whose frame is designed for the rear seats of the tonneau construction, but in which only two front seats are installed.n. See the extract.n. A West Indian helmet-shell, Cassis tuberosa.n. Something having the shape of a turtle's back.n. An arched protection erected over the upper deck of a steamer at the bow, and often at the stern also, to guard against damage from the breaking on board of heavy seas; a whaleback.