The herb known as Athelas in the Sindarin tongue is a powerful healing agent when correctly prepared, and a moderately effective one if less correctly prepared. It is common in Middle Earth, especially in those territories that form (or once formed) the kingdoms of Arnon and Gondor, having originally been carried there from Numenor.
For such a useful herb, its properties had been largely forgotten by all except the Dunedain Rangers of the northern wilderness by the end of the Third Age. It was the healing agent of choice for Aragorn, and no doubt was incorporated into the knowledge of Gondor, Rohan and their allies after his ascension to the throne as King Elessar.
In Westron (the tongue of men), its name is translated as Kingsfoil, while in the Elven Quenya language, it is called Asëa Aranion.