A druid is a type of seer, magician, or priest in the ancient Celtic civilization.
In Dungeons & Dragons[]
In Dungeons & Dragons, a druid is a type of specialty priest, distinct from the cleric. Druids concern themselves with nature, ancient religion, and balance. The druid was mentioned in the Greyhawk supplement, and became a player character class in 1976's Eldritch Wizardry.
In BECMI Dungeons & Dragons, a neutral cleric, upon reaching 9th level, could elect to become a druid, forsaking civilization and metal implements and gaining access to new spells.
An elf druid from Dungeons & Dragons 3rd edition.