The Dúnedain "west-men" [ˈduːnedaɪn], singular Dúnadain, were the Men of Númenor and (especially) their descendants who peopled Middle-earth in the Second and Third Ages.
The Third Age marked the beginning of the waning of the Dúnedain, in which their gifts of wisdom, nobility, and long life were slowly withdrawn due to the Downfall of Númenor and their mingling with lesser men.

In the beginning of their history, the Dúnedain were blessed with a lifespan thrice the life of lesser men, yet this ever-diminished over the course of the Third Age.