Katra was a term that was used by the Vulcan race.
This word was the name the Vulcan language gave to their soul or spirit as well as the essence of their memories and life. The term katra was a word that no one had ever been able to translate with any degree of precision. It was not exactly a soul or a personality, it was more than a memory but less than a living being. (TOS novel: Sarek) Typically, when a Vulcan neared death, they transferred their katra into the mind of another in order for it to be interred in the Hall of Ancient Thought at Mount Seleya on the Vulcan homeworld. Once there, the mind was placed within a katric ark where it resided in peace within the chambers of the Hall. (TOS - Vulcan's Soul novel: Exodus) Similarly, the acolytes of the Kolinahr ritual who resided at the Kolinahru Monastery were known to have a chamber that contained the minds of former High Masters of the Kolinahru. (Last Unicorn RPG module: The Way of Kolinahr: The Vulcans) In addition to this, there was the Fal-lan-tral near Seleya where individuals met with the Tral Katra in order to meet with the ancient minds of deceased Vulcan's. (TOS novel: Avenger)
Upon the transference of another's katra, the other individual often benefited from the experiences of that person's mind and memories. However, as the katra was capable of resisting the transfer, the procedure was not without risks and thus needed to be performed according to a specific ritual. This capacity to pass on a katra to another was not just limited to Vulcans and other species were capable of having a Vulcan mind transferred to them. In non-Vulcans, the experience was similar to those suffered from multiple personality disorders. Among Humans, they led to a severe shock upon the transfer as their nervous systems were quite vulnerable to the procedure. The restoration of a katra was normally done by a Vulcan priest who had experience in the use of katras. (ENT episodes: "The Forge", "Awakening", "Kir'Shara")
A Vulcan's katra lives on after the death of the physical body, and can be released into the mind of a living host being to either communicate with or control the host. Among the Vulcans who are known to have done this include the venerated Surak, the insane Zakal, and the Starfleet cadet, T'Priell. (TOS movie: Star Trek III: The Search for Spock; ENT episode: "Awakening"; TOS - The Lost Years novel: The Lost Years; SA comic: "Origins") Typically, a Vulcan who held the katra of another were known as a Keeper. (TOS novel: Sarek) Whilst this was the case with those Vulcans who had died, it was considered dangerous to mind meld with a disembodied mind; one of those placed within a katric ark. The only time it was considered a necessity to engage in such an act was in the gravest of emergencies as the outcome could lead to madness. (TOS novel: Star Trek V: The Final Frontier) Those individuals that housed a katra in their mind against their will were known as Val'reth. (VAN novel: Harbinger) It was also possible to restore a Vulcan if they had died if their katra had survived through the use of the Fal-tor-pan ritual. (TOS movie: Star Trek III: The Search for Spock)
The body of all living beings were noted to have katra points and stimulation of these regions helped in the promotion of healing. Furthermore, it was these locations that a Vulcan touched in order to create a mind meld. (TOS novel: Avenger) Humans were known to not possess a katra and thus their minds were not capable of being held by a Keeper. (TOS novel: Sarek)
The knowledge of the katra dated back to the time before the Time of Awakening when the Adepts of Seleya guarded the Hall of Ancient Thought and allowed travelers to pass on the mind of a katra into an Urn of Memory where they were preserved by the priestesses of the mountain. The disciple of Surak called Varen died on a pilgrimage to Mount Seleya and Surak carried his katra within his mind where it was interred at the Hall of Ancient Thought. (TOS - Vulcan's Soul novel: Exodus) During the time of the Vulcan-in-Exile when a large group of Vulcan's departed their Mother World on the Great Ships, the engineer Karatek had the katra of T'Kehr Rovalat transferred into his mind before the elderly Vulcan's death at the hands of the Providers. (TOS - Vulcan's Soul novel: Exiles)
However, by the 22nd century, the concept of the katra became a controversial view amongst the Vulcan race with the majority believing it to be nothing more than a myth. Several studies were conducted on katric arks recovered at the P'Jem monastery with a mind meld attempted but there was no evidence to suggest that suggested the existence of the katra.
The most notable example of the katra was that of the Vulcan philosopher Surak. Though he had died in the 4th century, his katra had survived the centuries until the year 2154 where it was transferred into the mind of Syran who led a revolution against the Vulcan High Command in order to restore Surak's true teachings. After Syran died in a sandstorm, he transferred Surak's katra to United Earth Captain Jonathan Archer who was successful in finding the Kir'Shara and restore Surak's original teachings. Surak's katra was later transferred to a Vulcan priest after the rise of T'Pau and re-organization of the Vulcan government. (ENT episodes: "The Forge", "Awakening", "Kir'Shara")
At some point, the katra of High Master T'Rea was placed within the Hall of Ancient Thought shortly after her death though the separation of her katra from her body was done hastily whereupon it was placed with the other vrekatras. A young Sybok later broke into the Hall and mind melded with the katra of his mother but was interrupted by High Master T'Sai. Rather than be interned again within the Hall, T'Rea's katra embraced true death leaving her mind silent within the Hall. (TOS novel: Star Trek V: The Final Frontier) Spock sensed the over-400 katra "scream", as the Vulcan crew of USS Intrepid met its demise in 2268. (TOS episode: "The Immunity Syndrome") A month later in the same year, Spock's own katra temporarily dwelled within Christine Chapel's consciousness. The two gained a mutual insight into one another, as well as their common affection. (TOS episode: "Return to Tomorrow")
In 2269 his katra is once again divided but made whole, courtesy of the clone Spock 2. After a Vulcan mind-meld was performed, they compared one another's logic. Both Spocks agreed that this procedure could benefit Doctor (Five) Stavos Keniclius' pursuit of using his genetic engineering skills for good. (TAS episode: "The Infinite Vulcan") A year later, when the Hydrillian sector artifacts released entities that turned the inhabitants of the planet Vulcan insane, T'Reth of Rh'Iahr intense desire to protect her son Skel led to her katra being ejected from her body and being placed in her childs mind which sought to protect him over the years he grew up. (TNG novel: Possession)
Spock once left his katra in the care of Dr. Leonard McCoy before Spock was killed saving the USS Enterprise from Khan Noonien Singh in the Mutara Nebula in 2285. After Spock's miraculous resurrection on the Genesis Planet in 2286, Spock's katra was restored to his own mind. For a brief time, portions of Spock's katra remained in the mind of McCoy, once enabling the doctor to engage in a mind-meld with a patient. (TOS movie: Star Trek II: The Wrath of Khan; TOS movie: Star Trek III: The Search for Spock; TOS comic: "The Last Word")
After the death of Amanda Grayson, Ambassador Sarek lamented that she was not a Vulcan or he would have been able to had saved her katra and intern it within the Hall of Ancient Thought. (TOS novel: Sarek) In 2373, a disguised James T. Kirk began manipulating the katra points on the body of a dying Teilani in order to help promote healing with the Tranni leaves. He accomplished this by touching her forehead and cheeks which led to Christine MacDonald to suspect he was a Healer. In the same year, Spock and Kirk journeyed to the Fal-lan-tral in order to make peace with the katra of Sarek. (TOS novel: Avenger)