H.S.M. Coxeter showed that a group Gamma is a finite reflection group of an Euclidean space if and only if Gamma is a finite Coxeter group. In this paper, we define reflections of geodesic spaces in general, and we prove that Gamma is a cocompact discrete reflection group of some geodesic space if and only if Gamma is a Coxeter group. (C) 2005 Elsevier B.V. All rights reserved.