Prophetic dreaming

"Well, it definately could be one of your prophetic dreams, or it could just be the eternal mystery that is your brain"

- Rupert Giles talking about a dream Buffy Summers had.

A prophetic dream was one of the main powers possessed by the Slayer.

Description
Slayer would, on occasion, experience dreams that foretold future events, whether it was a threat that's imminent or in years to come, if it was long term or an easily dealt-with situation or if it wasn't a threat anyway. These dreams could consist of a bizzare situation hinting the uncoming events or could just show the actual event itself, and were not strictly precognitive as Slayers, both Potential or activated, would experience dreams of the lived of past Slayers. Two existing Slayers would also form a pyshic link, allowing them to share dreams.

Buffy Summers
Buffy Summers has experienced prophetic dreams time-to-time, and the size of the threat would vary.
 * During her first day at Sunnydale High in 1997, before waking up, Buffy dreamed of the various experiences she eventually faced in future, including books about the Harvest, the last member of the Brotherhood of Seven, the origin of Moloch the Corrupter and prescene of the Master.
 * During her 17th birthday, Buffy experienced a dream set at the Bronze between her and her lover Angel, joined by  her mother, Willow Rosenberg and completely cured Drusilla, revealing that Drusilla has been restored even though she was presumed dead. In the dream, her mother asked her if she's ready just before Buffy walked over to meet Angel on the dance floor, both happy to see each other, symbolizing their growing feelings and intimacy. Drusilla then appeared from behind Angel and staked him, ending the dream. Buffy experienced another dream the same night, now involving featuring Angel being decapitated by Drusilla at the factory. These dreams forshadow the loss of Angel's soul along with Drusilla's involvement in making it happen by causing the chemistry between Buffy and Angel to ignite due to the pressure and co-dependence put on them with her plan to awaken the Judge. The following day, after the loss of Angel's soul, Buffy experienced a third dream which consisted of Buffy and Angel making love, only to move into a cemetary with Angel (now Angelus) and Jenny Calendar, revealing that she tried to keep Buffy and Angel away from eachother to prevent the loss of his soul.
 * While unconscious in hospital, Buffy shared a prophetic dream with Faith Lehane, who was in a coma at the time. Both Slayers were inside Faith's apartment and shared a dialogue that hinted at the arrival of Buffy's little sister Dawn represented by a cat, as Faith claimed "Little Miss Muffet counting down from 7-3-0", 730 days being 2 years which was the time before Dawn's arrival before this dream.
 * Buffy experienced a dream while asleep during Psych class. In the dream, Maggie Walsh had Buffy lie down in front of the whole class before being kissed by Riley Finn. Just as they embraced, the room went dark. Buffy walked through the halls of UC Sunnydale to find a little girl holding the Gentlemen's box and singing a disturbing laluby describing the actions of the Gentlemen. Buffy then saw one of the Gentlemen embodied in Riley. The dream forshadowed the arrival of the Gentlemen in Sunnydale while Maggie's speech about communication represented the lack of communication between Buffy and Riley to their secret lives.
 * Buffy dreamt of her now deceased mother, Joyce, taking to her while Buffy was in the middle of research. Joyce told her to get some rest, as Buffy was without sleep for days, causing her to be beaten to a bloody pulp by the Turok-Han vampire during their first encounter.
 * During 2002-2003, Buffy was in conflict with the First Evil and in the meantime, was protecting and training the remaining Potential Slayers. In the dream, Buffy was walking inside her home only to be attacked by the First Slayer, telling Buffy "It is not enough!". This represented how Buffy had to avasive actions if she wisheded to deal with the First's threat, leading her to confront the Shadow men.

Faith Lehane
Faith has had dreams, mostly involving her fellow Slayer Buffy.
 * Faith was struck in a coma after being stabbed by Buffy. She shared a dream with her at the place of where they fought where Faith hinted at Dawn's arrival before looking at a bunch of boxes. Buffy wondered if they'll be packed up, but Faith says they're here to stay, meaning Buffy must take on the role as a full time Slayer since Faith was then comatose.
 * The day Faith awoke from her coma, she experienced two prophetic dreams. This included her and Buffy making a bed, again hinting Dawn's arrival. It ended with Buffy's hand holding onto the knife that she used to stab Faith, sticking in Faith's stomach, showing how Faith views Buffy as a murderer. The next involved Faith having a picnic with Mayor Wilkins, who she viewed as a father. Buffy then interrupted the picnic by slashing the Mayor with the knife. Faith, now dressed in her street clothes opposed to the cardigan she was wearing before, ran away from Buffy who followed her into the cemetary. It ended with the Slayers falling into an open grave, fighting to the death, with Faith emerging from the grave victorious, ending the dream and her coma.

Appearances

 * "Welcome to the Hellmouth"
 * "Surprise"
 * "Innocence"
 * "Graduation Day, Part Two"
 * "Hush"
 * "This Year's Girl"
 * "Bring on the Night"
 * "Get it Done"