The Trill are a humanoid species native to the planet Trill. A small percentage of the Trill population co-exists with a sentient symbiotic organism known as a symbiont inside their bodies. The resulting joined Trill have personalities which are a synthesis of the two beings including the memories, and to some extent the personalities, of the previous hosts of the symbiont.
Most Trill are distinguished by two rows of spots going down each side of their body, from forehead to toe. One of the main neurotransmitters in the Trill brain is isoboramine. Joined Trill are also extremely allergic to insect bites; the biochemical connections between the host and symbiont cannot tolerate the reaction caused by the insect's venom. One Trill peculiarity is that they are known for having cold hands.
The few Trill that are given a symbiont are typically joined in their early to mid-twenties. The physical process of being joined is irreversible. Once joined, the host and symbiont are dependent on each other after 93 hours. If the symbiont is removed from the host, symbiont and host will die within hours, even if they are otherwise healthy, unless they are re-implanted with another host or symbiont.
Despite the relatively unusual nature of being a joined species, that aspect of their culture was widely unknown until 2367, when the Odan symbiont had to be given a new host during the middle of tense negotiations on Peliar Zel. Such a public example of the dual nature of the Trill brought to the forefront what had previously been a very private matter to Trill for several millennia.
In contrast to the greater revelation of their nature, the Trill are not a secretive species. To them, the joined nature of their culture is normal and not something they would think to comment on without prompting. Indeed, joined Trill can be seen as particularly genial and many have served the Federation as distinguished ambassadors, including Odan and Dax.
Jadzia Dax joked that Trill don't look for romance the way Humans do. They consider it quite a nuisance and view it as a weakness of the young. Although a Trill host may have romantic feelings on occasion, it is the symbiont's wish to live on a higher plane and to try to rise above those sorts of temptations.
Trill law forbids reassociation between subsequent hosts of joined persons whose symbionts were romantically involved in their previous hosts, as the main purpose of the transfer of symbionts is to experience new things in life. Trill who are found guilty of re-association are expelled from Trill society, and their symbionts die with their current host.
On average, only 300 symbionts are available for hosting each year. Because there are many more humanoid Trill than symbionts, prospective hosts are weeded out by a demanding selection procedure, overseen by the Symbiosis Commission. The competition for the few symbionts is fierce and attracts the brightest and most highly motivated of Trill society. Often the would-be hosts excel in their chosen fields and it is not uncommon for them to hold several degrees or distinctions prior to their joining. Prospective hosts may eventually become initiates under the supervision of a field docent, a joined Trill who evaluates the prospective host's suitability for joining and makes a recommendation to the Commission. A negative recommendation usually means the initiate is cut from the joining program.
Common belief in Trill society holds that only one in 1,000 Trill make acceptable hosts. In fact, this figure is vastly understated, and nearly half of the Trill population is capable of being joined. The myth is perpetuated very carefully, though, in order to avoid the widespread chaos which would arise if the information were made public, since the symbionts would become, essentially, objects to be fought over, as people fought to gain the few prized symbionts.
Unless the joined Trill objects, a prospective host may request a specific symbiont. If a host is weak, the personality of the symbiont will overwhelm it. The joined Trill has several tools at their disposal for dealing with various aspects of their previous hosts. The telepathic ceremony of zhian'tara allows a current host to divest the symbiont of the personality and all the memories of a previous host, which are temporarily hosted in volunteers. The transferring process is performed and supervised by a symbiont Guardian, an un-joined Trill telepath. The ceremony creates a chance for closure by having the new host address the previous hosts directly as a means of distinguishing their voices and cementing the sense of finality of its latest transition. Similarly, the Rite of Emergence can focus the voice of a single previous host among the memories of the other, allowing for a more direct converse in times of need for the current host.
The zhian'tara ritual is roughly similar to the Vulcan fal-tor-pan ritual, since both enable the respective species to perform a synaptic pattern displacement, the transfer of what could be considered as a soul (katra, pagh, etc.).