The Vulcan mind meld was a telepathic link between two individuals, allowing for the exchange of thoughts, thus in essence allowing the participants to become one mind. (TNG: "Sarek") It was a psionic technique for "synaptic pattern displacement". Normally it was employed only by Vulcans. It was a deeply personal thing, part of the private life, and generally not used on aliens, although cases were known where the mind meld was initiated between a Vulcan and a non-Vulcan. (VOY: "Meld", TNG: "Sarek") Those in a meld shared consciousness in a kind of gestalt. (DS9: "The Passenger")
Physical contact was required. The initiator placed the tips of his fingers at key locations on the head of the other participant. This allowed him to place pressure on key nerves and blood vessels to facilitate the link.
The fact that most Vulcans who performed the meld touch the other participant in different manners, reciting varying formulas, implies that it was only ritual behavior, helping to concentrate on the task, leaving mere physical contact being the ultimate prerequisite to initiate the meld. If successful, the link resulted in a merging of both minds, essentially creating a single consciousness in two bodies.
It is noteworthy that the meld could also be used by its initiator to probe another person's mind, while the melder him/herself shielded his/her mind from being read by the other participant, resulting in a rather one-sided psionic contact. (Star Trek VI: The Undiscovered Country) Only the subject in physical contact with the initiator participated in the meld; others in the vicinity are not affected.
Some species are immune to Vulcan mind meld and can resist it, shielding their mind from the attempted probing, including Cardassians, who can achieve this thanks to their intense mental training since childhood. (DS9: "The Maquis, Part II")
Melding carried some risk. It could be physically debilitating for both parties. The pressure changes could potentially aggravate existing conditions. The melding resulted in some loss of identity, and could be difficult to break, especially when the subject's mind was powerful or dynamic. Aftereffects could be treated with the druglexorin.
At the conclusion of the meld, each mind retained some knowledge of the other.
It was possible to force a mind meld on an unwilling subject, but most Vulcans would be loath to do so, unless under the most dire circumstances. An exception may be the Vulcans of the Mirror Universe, whose ethical constraints, shaped by the savagery of their environment, were far different.
During the 22nd century, mind melding was believed to be an ability only a minority of Vulcans were born to. Because of the apparent intimacy of melding, Vulcans during this era considered it a deviant practice defiant to the ancestral teachings of their society. As a result, other Vulcans considered those who were natural "melders" to be outcasts. (ENT: "Stigma")
An improperly trained melder who initiated a mind meld can cause a degenerative neurological disorder known as Pa'nar Syndrome to develop in the meld recipient. If left untreated, the disorder is fatal. Vulcan medicine of the mid-22nd century believed that there was no cure; however, the disorder was known in Surak's time, as was its cure: a corrective meld performed by an experienced melder. It is likely that the ignorance of this cure was due to the widespread prejudice against melders, a prejudice that was not shared by the Syrrannite group. (ENT: "Fusion", "Stigma", "Kir'Shara")
In 2266, on stardate 2715, Spock melded with Simon Van Gelder, then suffering from generalized synaptic damage, to learn whether Van Gelder's allegations about Tristan Adams were true, or delusions. Spock warned Van Gelder that the pressure changes could be dangerous. (TOS: "Dagger of the Mind")
The same year, on stardate 3192, Spock used what might have been a variation on a mind meld to convince a guard on Eminiar VII to open the door to the room where the landing party was imprisoned, facilitating their escape. (TOS: "A Taste of Armageddon")
Also in 2267, McCoy became the victim of an unwilling mind meld, when the mirror universe Spock forced information from him about the nature of the transporter accident that brought four USS Enterprise officers to the mirror universe. (TOS: "Mirror, Mirror")
The same year, Spock melded with Kirk, who was suffering from retrograde amnesia, consequential to the misuse of a Preserver artifact. Although Spock had some difficulty emerging from this meld, he was able to successfully restore Kirk's memory. (TOS: "The Paradise Syndrome")
In 2273, when the Enterprise was inside V'Ger, Commander Spock entered a section of V'Ger where it stored 3-D images of objects that it encountered through its travels. He entered this area via thruster suit and subsequently encountered a large representation of Lieutenant Ilia with a pulsing sensor on her neck. Believing it to have some special meaning, he used a mind meld on it. Spock was overwhelmed by the information from the mind meld, and was flung back unconscious towards the Enterprise. (Star Trek: The Motion Picture)
In 2371, Tuvok melded with crewmate Lieutenant Tom Paris after Paris was accused of the murder of Baneanscientist Doctor Tolen Ren. The meld allowed him to prove that Paris had been framed in an elaborate plot to provide the Banean's enemies, the Numiri, with critical information, when his analysis of the implanted memory revealed crucial anomalies that proved Paris couldn't have committed the murder. (VOY: "Ex Post Facto")
In 2372, Tuvok melded with the sociopath BetazoidLon Suder in an attempt to gain a better understanding of Suder's violent impulses. While the meld provided Suder with a measure of emotional control, it also temporarily released Tuvok's more violent and primal urges. (VOY: "Meld")
Lon Suder referred to the mind meld as "that Vulcan thing where you grab someone's head"
In 2374, Tuvok attempted to perform a mind meld with Kes in order to give her stability when her mental powers began to grow out of control. Later that year, he melded with an alien named Guill in an investigation to proveB'Elanna Torres' innocence. Guill was from a society where violent thoughts were illegal. Guill was a collector and trader of such thoughts. Tuvok proceeded with the meld under the pretense that only images would be exchanged, but he also grabbed and choked Guill in the meld to get him to submit. (VOY: "Random Thoughts")
In 2376, Tuvok performed an extended technique of the mind meld, the "Bridging of Minds", on Captain Janewayand Seven of Nine, forming a bond between the two individuals, who were otherwise unable to telepathically link. (VOY: "Unimatrix Zero")
Aelita used a mind meld for the first time. She mind melded with Yami Yugi.
Aelita used a mind meld to transfer the location of the Black Star Dragon Balls to Joey before shutting down the supercomputer. Chris managed to escape the virtual world, Aelita mind-melded with him to explain about she taken the Four-Star Dragon Ball using Victoria's Secret Compartment while Noah stealing other two. Chris decides to convince Noah to had a change of heart. Téa is cheering Yugi on the whole time while Aelita is booing Yami Marik. She discovers that Marik took over Téa again. Telling him that Téa is no longer under Marik's control, she used a mind meld on her. Marik was overwhelmed by Aelita's visions from the mind meld, and was flung away from Téa to Joey. Due to the meld, Téa finally remembers about what just happened.