Seer is a term used to refer to beings with the psychic ability to foresee future events or become aware of contemporary ones without the mediation of the regular five senses.
Seers obtain information of these events whether by mental flashes with sensory information (visions), through dreams (premonitory dreams) or through intuition alone.
In the supernatural black market, the eyes of seers are highly valued, as they can be used as amulets in which the seer's powers are supposedly stored.
The Enochian protection runes protect the bearer from being viewed remotely from them.
While not seers per se, Slayers do experience premonitions, not just about the lives of previous Slayers, but also about future events as well.