Tessellation is the process of splitting a polygon (or surface) into a set of simpler and more tractable (controllable) polygons (e.g. triangles or quadrilaterals). In case of splitting a polygon into triangles, the process is called Triangulation. (1)
(1) Akenine-Moller, Tomas (2008) Real-Time Rendering, 3rd Ed. Wellesley, Massachusetts: A K Peters, Ltd.