We lay the foundations for the study of relatively quasiconvex subgroups of relatively
hyperbolic groups. These foundations require that we first work out a coherent theory
of countable relatively hyperbolic groups (not necessarily finitely generated). We
prove the equivalence of Gromov, Osin and Bowditch’s definitions of relative
hyperbolicity for countable groups.
We then give several equivalent definitions of relatively quasiconvex subgroups in
terms of various natural geometries on a relatively hyperbolic group. We show that
each relatively quasiconvex subgroup is itself relatively hyperbolic, and that the
intersection of two relatively quasiconvex subgroups is again relatively quasiconvex.
In the finitely generated case, we prove that every undistorted subgroup is relatively
quasiconvex, and we compute the distortion of a finitely generated relatively
quasiconvex subgroup.
