A seer is one who perceives what is hidden, whether the future, the state of the dead, or the cause of a present affliction, and the office is defined by a claimed faculty of perception.
A faculty, not a technique
The seer is set apart by an inward power of perception, a gift of sight into what is concealed, rather than by the mastery of a technique, which is the distinction that separates the seer from the diviner: the diviner reads an outcome through a method, the casting of lots or the inspection of signs, while the seer simply sees. This claimed faculty may extend to the future, to the world of the dead and spirits, or to the hidden cause of an illness or misfortune, and it is on the strength of it that the seer is consulted.
A widespread figure
The seer appears across a great many traditions, from the prophets and visionaries of the scriptural religions to the seers of tribal and folk cultures, and the office is generally recognised rather than appointed, the standing resting on a gift held to be given rather than acquired, though it may be developed by discipline. Because the faculty is inward and cannot be simply demonstrated, the standing of a seer depends on acceptance and on the record of what the seeing has revealed. The figure overlaps with the prophet, the visionary and the medium, all sharing the claim to perceive what ordinary sight cannot, and the seer is the general name for one whose authority rests on that claimed inner perception.