Dax (Star Trek)
Dax is the name of a fictional being in the Star Trek universe, a Trill symbiont. It's a very long-lived slug-shaped sentient worm-like life form that lives inside the bodies of a succession of Trills, a race of humanoid aliens. After being surgically implanted, the symbiont merges with its host by a process called "joining" after which it becomes an essential part of the host's body for the rest of the host's life. In the joining, the personalities of both symbiont and host merge, essentially creating a new and distinct personality. This new joined Trill will take the symbiont's name as his/her new surname. For example, after joining with the Dax symbiont, Ezri Tigan became Ezri Dax. Each successive host has access to the memories of the previous hosts as well as their accumulated skills and work experience.
Two of Dax's hosts, Jadzia Dax and Ezri Dax, appear as major characters in Star Trek: Deep Space Nine. Others are only seen episodically – in few flashbacks and when taking over others' bodies in the DS9 season 3 episode "Facets". They play more important roles in some non-canon spin-offs such as Star Trek: The Human Frontier and The Lives of Dax.
Throughout the franchise's timeline, Dax has been joined with four men and five women, living for more than three hundred years in total. Dax is a part of some of the most important events in the Star Trek history like signing of the Khitomer Accords and discovery of the Bajoran wormhole.