Dúnedain (plural of Dúnadan which is the singular form) is a term used in Middle-earth for the Men of Númenor and (especially) their descendants in Arnor and Gondor after the Downfall in II 3319. The Rangers of the North are the largest group of Dúnedain in Northwestern Middle-earth by the time of the War of the Ring. Their main bases of operations are the island of Tinnudir in Evendim and Esteldín in The North Downs.
The word is Sindarin, and means "Men of the West", from "dún" ("west") and "edain" ("men") (edain in turn is from Quenya "atani", meaning "The Second" since Men came after Elves).
Dúnedain were considered "High Men," a race of supposedly nobler stock than the "Middle Men" that compose the other peoples of Middle-earth or the "Men of Shadow" who are loyal to the Dark Lord Sauron. The kings of the Dúnedain claimed kinship with the Half-elf Elros (brother of Elrond) and the other members of the race were granted extended life spans by the Valar. Dúnedain once commanded the greatest kingdoms of Middle-earth, but after the destruction of Arnor in its civil wars and its conflict with Angmar, and after the death of the last king of Gondor, their glory has been diminished.
The Dúnedain who remained in the north at the end of the Third Age were far more alike to their Númenórean ancestors than those in the south, having intermarried less freely with the "Middle Men", tall and grey-eyed, with dark hair and pale skin. The Chieftains of the Dúnedain, continuing the line of Isildur after the fall of Arthedain, still enjoyed a longer lifespan than most (assuming death in battle did not find them first).
In the Third Age, most live as Rangers in the wilderness or in small, isolated villages in the former lands of Arnor. However, the Dúnedain and their allies live in hope for the eventual return of the king and a remaking of the broken royal line.