Jadzia Dax (left) and the symbiote's successor host, Ezri Dax, from The Lives of Dax book cover

Dax is a fictional character on the television show Star Trek: Deep Space Nine.

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 including the accumulated skills and work experiences. Two of Dax's hosts, Jadzia and Ezri, were regular characters on Deep Space Nine. Dax has had a total of nine hosts:

1. Lela Dax; embodied into Kira Nerys during Jadzia's zhian'tara
2. Tobin Dax; embodied into Miles O'Brien during Jadzia's zhian'tara
3. Emony Dax; embodied into Leeta during Jadzia's zhian'tara
4. Audrid Dax ; embodied into Quark during Jadzia's zhian'tara
5. Torias Dax; embodied into Julian Bashir during Jadzia's zhian'tara
6. Joran Dax (Jeff Magnus McBride in episode "Equilibrium" and Leigh J. McCloskey in "Field of Fire"); embodied into Benjamin Sisko during Jadzia's zhian'tara
7. Curzon (Frank Owen Smith - episode "Emissary"); embodied into Odo during Jadzia's zhian'tara
8. Jadzia (Terry Farrell); and
9. Ezri (Nicole de Boer).

In an alternate future, a descendant of Jadzia's, Yedrin (Gary Frank - episode "Children of Time"), was shown to be hosting Dax.

Joran hosted Dax only briefly before he was found to be a murderer. The Symbiosis Commission subsequently attempted to cover-up their error, though was later discovered when a memory block began to fail, causing Jadzia Dax to recall memories of Joran.

In the second season episode "Invasive Procedures", the Trill humaniod Verad (John Glover) stole the Dax symbiont for a few hours. Verad was a rejected candidate for joining that took the Dax symbiont from Jadzia by force and had it implanted in himself. Dax was later returned to Jadzia (in a sense making Jadzia both the eighth and tenth hosts, though it is not generally counted this way) before the symbiont and host were intergrated.

Dax, son of Kang was a Klingon named after his godfather, Curzon Dax. He died at a very early age from a genetic virus given to him by an alien known as the Albino.

Hosts

Emony Dax

The third host of the Dax symbiont. Even before her joining, Emony was an excellent athlete and gymnast; her coordination improved after joining with Dax. She was on Earth in 2245, judging a gymnastics competition; while there, she met and had a relationship with a young Leonard McCoy.

Dax was also the name of a crewman aboard the USS Enterprise (NCC-1701-A) in the film Star Trek VI: The Undiscovered Country. A pair of magnetic boots used during the murder of Klingon Chancellor Gorkon were found hidden in his locker, but Dax's exceptionally large feet precluded his having worn the boots. Crewman Dax does not appear Trill and is not known to bear any relation to the Dax symbiont.