A polymesh is a 3D model that is composed of polygons. A polygon is a shape that is enclosed by a number of points connected by edges. The polygons are connected to each other to form a sort of net (or ‘mesh’) that defines the shape. The whole shape can be open, where some edges belong to only one polygon, or closed, where all edges belong to two polygons so that the mesh is watertight.
ZBrush uses polymeshes that are composed of quads (quadrangular or 4-sided polygons) and tris (triangular or 3-sided polygons).