Dax is a Trill[?] symbiont, a very long-lived slug-shaped entity that is hosted by a succession of humanoid Trills, a process called "joining". Each of the hosts has access to the memories of the previous hosts. Dax has had a total of ten hosts:
In an alternate future, a descendant of Jadzia's, Yedrin Dax, was shown to be hosting Dax.
wikipedia.org dumped 2003-03-17 with terodump