A Seer is a witch or a cunning person with the power to divine future, past and present events through extrasensory perception.
While all witches appear to have some degree of clairvoyance, a seer is a witch or a cunning person that is significantly more attuned with the psychic vibrations of the universe, and may access information through various techniques that are incomprehensible to the average sorcerer. Even a witch as powerful as Countess Von Marburg needed to rely on a ritualistic spell in order to obtain hidden information about Mary Sibley and the Essex hive. Whereas, a seer named Petrus, though considerably less powerful than the Countess, was able to learn about almost any event, future or past, through sheer will. Nevertheless, Petrus once expressed to John Hale how divination in not an exact art, because contrary to puritanical belief, the future is not set in stone and can always change.
Petrus: Petrus was an eccentric and free-spirited individual who strongly valued his privacy in the woods, accompanied by his familiars and various other animal companions. In spite of the fact that Petrus was a witch, and commonly acted as an emissary to the Essex hive, Petrus explained to John Alden, that he also provided help to the Indians on occasion. Nevertheless, Petrus was his own man who offered help to whomever sought him out. He expressed to John Alden, that he did not side with the witches nor the puritans nor the Indians; referring to himself as a "circle without sides".
Tituba: Tituba is a very powerful witch who obtained the power of divination shortly after the consumption of Petrus's eyes.
Mary Sibley: Concern yourself with one thing, Mr. Hale... Find out who broke our circle. Head back to the woods. Find the seer. His eyes were there.
John Hale: Petrus, really. You ought to wear a little bell.
Petrus: And you, Magistrate, need none. We heard you the moment you left the road. Come.
John Hale: I had a bit of trouble finding the place. Seemed to recall you having been farther north last time.
Petrus: Perhaps I was, magistrate Hale. Eyes are for seeing, not for being seen. That's why your kind come to me. You find me when you need me. So, what do you need?
John Hale: Someone broke our circle in the woods last night. We need to know who saw us.
Petrus: Ah. Full buck moon? Uh-huh. You were there that night, weren't you, little friend? Time to wake up. What did you see? Tell Mary Sibley we will find out who was in the woods last night, but it takes time.
The eyes of a seer are the source of their power. This was proven after Tituba consumed both of the eyes that were left behind by the recently murdered Petrus and immediately developed his second sight. Through this power, Tituba was able to discover the identity of the latest witch hunter in Salem as John Alden.
Seers are able to share their visions with others regardless of whether or not the person is a witch. This was proven after Mary Sibley sought the help of Petrus to discover the outcome of John Alden after she abandoned him in the woods. However, the vision that was seen by Mary was forged by the Indians, and while Mary believed the vision to be true, Petrus could sense that is was a lie.