Vates (Celtic: *wātis ´seer, sooth-sayer´; Greek: , ) were a class of seers and sacrificers among the ancient Gauls. Greek and Roman writers describe them as one of three specially honoured groups in Gaulish society, ranked beside the bards and the druids. They conducted sacrifices, foretold the future, and concerned themselves with the natural world. The account goes back to the lost Gaulish ethnography of the Greek philosopher Posidonius. It survives in three later writers, Diodorus Siculus, Strabo and Ammianus Marcellinus, who name and describe the class in terms that do not fully agree. The native word behind the Greek and Latin forms is reconstructed as Proto-Celtic *wātis, also continued by Old Irish fáith (´seer, prophet´).

Attestations: The Gaulish class is mentioned by Diodorus Siculus (1st century BC) and Strabo (1st century AD) and Ammianus (4th century AD), who all drew on the lost Gaulish ethnography of Posidonius (c. 135–51 BC). Of the three authors, only Strabo gives what appears to be its native name, under the Greek plural (). Diodorus, drawing on the same source, uses the Greek word for ´seers´, (). Ammianus, who depends on the historian Timagenes, has the Latinised form euhages, which the manuscripts and early editions also transmit as eubages.

How the three renderings relate, and which best reflects the Gaulish term, has been argued since the 19th century. Most scholars hold that Posidonius used a single Celtic word, which his excerptors glossed in different ways, so that Diodorus´s , Strabo´s and Ammanius´s euhages designate the same group. The form euhages is usually explained as a corruption or gloss of ouáteis, perhaps influenced by the Greek adjective heuageis (; ´pure, holy´). The suggestion that euhages is itself a Gaulish word, as advanced by Jean-Louis Brunaux, has been rejected by Bernhard Maier for want of a credible Celtic etymology.

Lucan and Pliny the Elder use the word vātēs in its ordinary Latin sense and do not attest the Gaulish word. In Lucan the word is applied to the bards, and Pliny uses it of the seers and healers grouped with the druids.

Etymology: The Greek word ouáteis () is derived from Proto-Celtic *wātis (´seer, sooth-sayer´), also continued in Old Irish fáith (´seer, prophet´) and in the Gaulish theonym Uatiounos (< wātio-mno-, ´who prophesies´). A related form *wātus (´poetic inspiration´) gave way to Gaulish uatus, Old Irish fáth and Welsh gwawd (´song, poem, satire´).

The same root appears in Germanic *wōđaz (´possessed, inspired, delirious, raging´), giving Gothic woþs (´possessed´), Old Norse óðr (´mind, wit, soul, sense´; also ´song, poetry´) and Old High German wuot (´frenzy´), together with the divine name *Wōðanaz (Old Norse Óðinn, Old English Wōden, Old High German Wuotan). Koch and Kroonen treat *wōđaz and *wātis as religious terms shared by Germanic and Celtic, going back to a common form *uoh₂-tós ~ *ueh₂-tus (´god-inspired´).

The Latin vātēs (´seer, prophet, poet´) corresponds closely in form and meaning to the Celtic word, but their relation is disputed. Michiel de Vaan and Ranko Matasović take them as cognates ultimately descended from a Proto-Indo-European word *(h)ueh₂-tis (´prophet, seer´). Conversely, Xavier Delamarre, Jan de Vries, Guus Kroonen, and John T. Koch regard Latin vātēs as an early loan from Gaulish.