Subgroups G0,G1,…,Gn of a finite group G such that (i) {e} = G0 and G = Gn, (ii) Gk is a normal subgroup of Gk+1 for 0 ≤ k < n, (iii) the quotient groups Gk+1/Gk, called composition factors, are all simple. {e}⊳A3⊳S3 is a composition series for S3 as A3/{e}≅C3 and S3/A3≅C2 are simple. See Jordan-Hölder Theorem.