According to the Ancient Greek writers Strabo, Diodorus Siculus, and Poseidonius, the vates (ουατεις) were one of three classes of Celtic priesthood, the other two being the druids and the bards. The Vates had the role of seers and performed sacrifices (in particular administering human sacrifice), under the presidence of a druid. Their role therefore corresponded to that of an Adhvaryu in Vedic religion. Celtic vates is continued by Irish fáith "prophet, seer," and ofydd in Welsh.
It is unknown whether the Latin and Gaulish usages are cognates, or if the former should be considered a Celtic loanword. The word may be derived from a PIE root "to inspire, spiritually arouse"; however that root cannot be shown to go back to Proto-Indo-European, since it is only certainly attested for Celtic and Germanic (though it may be present natively in Italic). Virgil uses the Latin vannus "winnowing fan" (from , compare Old High German wadal, modern German Wedel, with the same meaning, from ) for something borne about in the Bacchic festival, suggesting that the root may have had an ecstatic sense in Italic also.