Visions

Visions was a term used to refer to the ability to perceive future events through visual flashes.

A particular kind of visions was caused by a psychic link to the Powers That Be. They consisted of vague images, sounds, and occasionally scents which warned people of danger. The visions typically caused the user great pain, and only demons and half-demons could bear them without suffering any long-term damage.

"The" Visions
At one point a human girl named Tammy received the vsions from the Powers That Be. Unable to handle their power, Tammy eventually died when the visions blew the back of her head out.

At some point in the late 20th century, a half-demon named Allen Francis Doyle was granted the visions. They led him to the vampire-with-soul Angel, putting Angel on the path of his destiny. Shortly before he died, he transferred the visions to Cordelia Chase via a kiss. Cordelia, although initially ungrateful for Doyle's "parting gift", eventually came to view them as an asset. Initially they were killing her, but thanks to the demon Skip (and presumably Jasmine) Cordelia became part-demon which allowed her to bear the visions with no trouble. Also, unlike the half-demon Doyle, Cordelia no longer felt any pain at all with her visions.

When Cordelia died, she used her last day on Earth to put Angel back on track, and transferred one last vision to him as Doyle had given them to her. Angel received only one vision, which led him to bring down the Circle of the Black Thorn. Proceeding this, Angel has stated that his vision was a "one-shot deal."

Following Angel's vision, a gang of vampires found a demon whose innate powers allowed for the visual projection of what they believe are the Powers' visions. Using these, they tracked Team Angel outside the Hyperion Hotel during the showdown with the Senior Partners' army and used the opportunity to sire Charles Gunn. Gunn, as a vampire, fed on the demon to acquire its visions. However these visions did not come from the Powers That Be, as they would have severed the connection to prevent the visions to be used for evil. Wesley later revealed that these visions, while accurate, came in fact from the Senior Partners.

The current host for the visions, if indeed there is one, is unknown.

Other instances of visions

 * Slayers are capable of perceiving future events in their dreams.
 * In order to blackmail Angel into breaking Billy Blim out of the prison dimension where he was trapped, Lilah Morgan hired a psychic demon to tap into the "celestial pipeline" through which the Powers sent their visions and send visions of his own; however, these visions had the side effect of inflicting physical damage based on what Cordelia saw.
 * Lorne and other members of the Deathwok Clan could see the futures of others by reading their auras.