Quad

In polygon-based modeling, a quad is any face that contains exactly 4 vertices. Quads are often preferred in many areas of 3D modeling as they're shaded consistently across multiple programs, and will work with most 3D algorithms. For rig meshes in particular, quads make the process of rigging much easier, and will result in predictable deformation.

Quads are commonly used in [Subdivision Modeling|subd modeling], where having quad faces on the base model will keep the final subdivided version clean and avoid issues with shading.