Dúnadan (Sindarin for "Man of the West") is the singular form of the noun. The plural term is Dúnedain.
Most people first encounter this word in The Lord of the Rings, in the scene just after Frodo Baggins wakes up in the House of Elrond (Book II, Chapter I: "Many Meetings"), where Frodo's uncle Bilbo Baggins uses the term as if it were general knowledge. This baffles Frodo, and allows Bilbo to give a little lecture later on.
'The Dúnadan', said Bilbo. 'He is often called that here. But I thought you knew enough Elvish at least to know dún-adan: Man of the West, Númenorean.'
He is of course referring to Frodo's guide Strider, whose real name is Aragorn, and turns out to be rather important to the story.