Norway was united to one
kingdom by
Harald Hårfagre (at least, he has been credited for it) after the
Battle of Hafrsfjord. The name is mentioned for the first time by
Ottar from Hålogaland (~800) in his meeting with
King Alfred of
England. He talked about "Nordvegr", meaning "North way" or "The way to the North". The name thus comes from the trade routes along the
Norwegian coast.
(Small digression to conclude: Ottar also has the first account of Sami populations in North of Norway)