Dovahkiin is the word dragon-born in
dragon tongue, it is used in the elder scrolls games, as a myth or legend until in "The elder scrolls IV - oblivion" where you help the dovahkiin, in "the elder scrolls V - skyrim" you ARE the dovahkiin.
The only NPC(s) that use the word dovahkiin instead of "dragon-born" are:
The greybeards, paartharax, mirmulnir (this is the first dragon you fight, they say "noooooooo, dovahkiin!" When defeated, so that is why it counts) and Alduin, the main terror in skyrim's main quest.
Dovahkiins are naturally skilled in using shouts (or in the
dragon tongue, thu'ums) such as:
Unrelenting force, fire breath, become ethereal and my personal favorite, slow time. Shouts are made up of 3 words of power that are in the
dragon tongue.
