An oracle, also known as a farseer, is typically a person (but possibly some other creature) considered to be a source of wise counsel or prophetic opinion; an infallible authority, usually spiritual in nature. In the Witcher saga, oracles tend to be described as "slobbering" and "half insane"; the result of gifted individuals called "sources" who failed to master their own innate powers. Ithlinne is described as an oracle ... but salivating habit details are not provided. They can also be created, such as women studying at the Temple of Melitele to become oracles.
Objects can also have oracular qualities. Nehalenia's Mirrors were said to possess such properties.