x1 is a regular polyhedra/polygon/polytope with the Schläfli symbol {x2, x3, x4, …}
also "x1 is a regular polytope". Polytope is a collective term for polygons, polyhedra and other dimentional shapes. "x1 is a regular polytope with the Schläfli symbol x2" is also valid if you want to use vectors. For Coxeter's extention, use a matrix. Cf. kubli