The Viking-era language that became the ancestor of modern Danish, Swedish, Norwegian and Icelandic was Old Norse.
Old Norse was a North Germanic language spoken across Scandinavia and in Norse settlements overseas during the Viking Age and the medieval period. It was written first in runes and later in the Latin alphabet, especially in Iceland and Norway.
The language had several regional varieties. Old West Norse was associated mainly with Norway and Iceland, while Old East Norse was spoken chiefly in Denmark and Sweden. Their differences were real, but speakers remained closely connected through trade, migration, and political ties.
Old Norse also influenced languages outside Scandinavia. English gained words such as “sky,” “egg,” “law,” and “they” through contact with Norse speakers in areas including the Danelaw. Modern Icelandic preserves especially extensive features of the older language, although it is not identical to Viking-Age speech.