Definite: Face :-)

Assimp.mesh.h

/** @brief A single face in a mesh, referring to multiple vertices. 
 *
 * If mNumIndices is 3, we call the face 'triangle', for mNumIndices > 3 
 * it's called 'polygon' (hey, that's just a definition!).
 * <br>
 * aiMesh::mPrimitiveTypes can be queried to quickly examine which types of
 * primitive are actually present in a mesh. The #aiProcess_SortByPType flag 
 * executes a special post-processing algorithm which splits meshes with
 * *different* primitive types mixed up (e.g. lines and triangles) in several
 * 'clean' submeshes. Furthermore there is a configuration option (
 * #AI_CONFIG_PP_SBP_REMOVE) to force #aiProcess_SortByPType to remove 
 * specific kinds of primitives from the imported scene, completely and forever.
 * In many cases you'll probably want to set this setting to 
 * @code 
 * aiPrimitiveType_LINE|aiPrimitiveType_POINT
 * @endcode
 * Together with the #aiProcess_Triangulate flag you can then be sure that
 * #aiFace::mNumIndices is always 3. 
 * @note Take a look at the @link data Data Structures page @endlink for
 * more information on the layout and winding order of a face.
 */