“Helicoid,” used as both an adjective and a noun, comes from the Greek helikoeidēs, meaning “of spiral form.” The thread on a screw is a perfect example of a helicoid. It’s a shape formed by moving a straight line along an axis as it turns. There are human-made helicoid structures, such as a spiral staircase, but you can find many examples of helicoids in the natural world, too.