turtleback

noun

1. Anything having the shape of a turtle's back (that is, its shell).

   “Up he went on concrete stairs that were eroded to turtlebacks and slippery with evening's frost […]”

2. A convex deck at the bow or stern of a vessel, designed to shed seawater quickly.
3. An armor layout with an armored deck which slopes downwards towards the sides of the ship and connects to the lower edge of the main belt armor, designed to deflect shells striking the ship on trajectories close to horizontal.
4. A primitive stone celt of a form suggesting the back of a turtle.
5. A library binding of a mass market paperback with a generic hardcover.

   “When you order Turtleback Books from Baker & Taylor, you can be assured you get Turtlebacks!”

6. Any plant of the genus Psathyrotes of annual and perennial forbs and low subshrubs native to dry areas of southwestern North America.