QUAD

(quadrilateral) (n.) A closed, four-sided polygon. Quadrilaterals, or quads, can be planar (in which case all vertexes lie on the same plane) or non-planar. With non-planar quadrilaterals (sometimes called bow ties, because of their shape), calculating orientation and lighting is more difficult and thus many systems tessellate quads to triangles, which are definitively planar.