"Certainly I knew, Minerva. But one does not parade the fact that one is All-Knowing. I frequently act as though I am not possessed of the Inner Eye, so as not to make others nervous."
— Sybill Trelawney
Sybill Patricia Trelawney is the Divination professor at Hogwarts. She is the great-great-granddaughter of Cassandra Trelawney, a very gifted, very famous Seer (OP37). Hermione considers her to be nothing but an old fraud, but Trelawney has her moments.
Trelawney, according Dumbledore, has made exactly two accurate predictions since coming to Hogwarts. In 1979, Dumbledore went to interview her at the Hog's Head in Hogsmeade. He was not particularly impressed until, to his surprise, she apparently fell into a trance and spoke these words:
"The one with the power to vanquish the Dark Lord approaches ... Born to those who have thrice defied him, born as the seventh month dies ... And the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not ... And either must die at the hand of the other for neither can live while the other survives ... The one with the power to vanquish the Dark Lord will be born as the seventh month dies..." (OP 37).
Severus Snape was eavesdropping on this conversation and he reported the first part of the Prophecy to the Dark Lord. Voldemort immediately began searching for this threat, and centered his attention on the child of Lily and James Potter (OP 37). A record of this Prophecy was kept in the Department of Mysteries ever since.
Many years later, in June of 1994, Trelawney again fell into a trance. This time she said:
"IT WILL HAPPEN TONIGHT. THE DARK LORD LIES ALONE AND FRIENDLESS, ABANDONED BY HIS FOLLOWERS. HIS SERVANT HAS BEEN CHAINED THESE TWELVE YEARS. TONIGHT, BEFORE MIDNIGHT... THE SERVANT WILL BREAK FREE AND SET OUT TO REJOIN HIS MASTER. THE DARK LORD WILL RISE AGAIN WITH HIS SERVANTS AID, GREATER AND MORE TERRIBLE THAN EVER HE WAS. TONIGHT... BEFORE MIDNIGHT...THE SERVANT...WILL SET OUT...TO REJOIN...HIS MASTER...." (PA)
This second prophecy came true that evening when Peter Pettigrew, who had been masquerading as a rat, escaped and went to Albania to find the vanquished Voldemort.
True prophecies those may be, but Trelawney doesn’t even remember making them. Her version of Divination is fortune telling. She likes to start off her classes by predicting the death of someone in the class. She teaches a variety of Divination methods including reading tea leaves and reading the crystal ball, all in a tower classroom that is overheated with a perfumed fire and filled with stuffed furniture and little tables. Harry hates her class and Hermione actually quit taking it in disgust. McGonagall has little use for Trelawney's brand of magic, and quite honestly Dumbledore doesn't expect much of it either.
Not everyone would be so protective. During the 1995-6 school year, Hogwarts High Inquisitor Dolores Umbridge inspected all the teachers' classes and teaching methods. She first put Trelawney on probation, then fired her. In a move probably meant to intimidate the rest of the Hogwarts staff, Umbridge further humiliated Trelawney by evicting her from Hogwarts with the students looking on. At that point, Dumbledore stepped in, announced that he had found a new Divination teacher, and that Trelawney could still consider Hogwarts her home.
The next school year Trelawney was teaching again, but had to share the position with Firenze, whom she peevishly resented, calling him "Dobbin" and referring to him as "the horse." Her complaints and warnings went unheeded by Dumbledore or anyone else in the school. At one point she was walking around with Tarot cards, talking about the Lightning-Struck Tower card, which was prescient indeed, but no one noticed.
During the Battle of Hogwarts, Trelawney found a good use for her stock of crystal balls. She used her wand to fire them from the balcony above the entrance hall, smashing them spectacularly onto the heads of Death Eaters. She flattened Greyback this way (DH36).
Mother: A muggle (Pm).
Father: A wizard (Pm).
Great-great-grandmother: Cassandra Trelawney, a genuine, gifted Seer (OP37).
Sybill is a Seer, although the ability is unconscious and unpredicatable.
Other canon notes and references
Professor Trelawney is the author of a book on divination called My Eyes and How to See Past Them (CC1.19).
Sybill: In ancient times a Sibyl was a prophetess who, in a state of ecstasy and under influence of Apollo, prophesied without being consulted (EM).
The American editor changed her name to Sibyll. J.K. Rowling wrote: "I preferred my version, because while it keeps the reference to the august clairvoyants of old, it is really no more than a variant the unfashionable female name 'Sybil'. Professor Trelawney, I felt, did not really qualify as a 'Sibyl'." (Pm).
Trelawney: An old Cornish name, "Tre" means homestead. There is a very small village named "Trelawne" in Cornwall near Looe and Polperro (it is also near a town called Crumplehorn).
The ancient surname reflects Sybill's reliance on her ancestry (Pm).
The fact that Professor Trelawney is descended from a seer named Cassandra is not insignificant. In Greek mythology (from which Rowling draws frequently and heavily), Cassandra was given the gift of prophecy, but then cursed so that no one would believe her predictions (source: Encyclopaedia Britannica). -BB
From the Web
An examination of Trelawney's predictions and their outcomes: