x1 is a regular polyhedra/polygon/polytope with the Schläfli symbol x2; x1 is a regular polygon with x2 number of sides.
also "x1 is a regular polytope". Polytope is a collective term for polygons, polyhedra and other dimentional shapes. Use a vector to express the Schläfli symbol. For Coxeter's extention, use a matrix. Cf. kubli