Short answers, pulled from the story.
The word Norseman first appears in English during the early 19th century. The earliest attestation given in the third edition of the Oxford English Dictionary is from Walter Scott's 1817 Harold the Dauntless.
Norsemen mainly invaded islands north and northwest of Britain as well as Ireland. These people established polities across Great Britain, Iceland, Russia, Belarus, France, Sicily, Greenland, Canada, and many other regions.
Harald Fairhair became the first King of Norway around the year 850 CE. He unified various Norwegian kingdoms before his death which occurred later in history.
Slavs called them Rus' while Byzantines knew them as Varangians. The name Varangian means sworn men within that historical context.
Modern use of the word viking has no particular basis in medieval usage either. In Old Norse and Old English the word simply meant pirate rather than describing organized raids.