There are multiple theorems named after the French mathematician Camille Jordan:
The Jordan curve theorem states that every simple closed curve has a well-defined "inside" and "outside"; Jordan's lemma is a bound for the error term in applications of the residue theorem; Jordan's theorem on group actions characterizes primitive groups containing a large -cycle; and The Jordan–Schur theorem is an effective proof (in terms of the degree) that linear torsion groups are virtually abelian.