The Collaborative International Dictionary of English v.0.48:

ionophore \i*on"o*phore`\ n. [ion + -phore] (Biochemistry)
   A compound that enhances transport of ions across cellular
   membranes, by binding them and carrying them across. As a
   class ionophores tend to be lipophilic in nature; each
   compound is usually specific for one or a small number of
   ions.
   [PJC]