The Anglo-Saxons were the population in Britain partly descended from the Germanic tribes who migrated from continental Europe and settled the south and east of the island beginning in the early 5th century. 

The Anglo-Saxon period denotes the period of English history after their initial settlement through their creation of the English nation, up to the Norman conquest; that is, between about 550 and 1066.

The term Anglo-Saxon is also used for the language, today more correctly called Old English, that was spoken and written by the Anglo-Saxons in England (and parts of south-eastern Scotland) between at least the mid-5th century and the mid-12th century, after which it is known as Middle English.

Old Norse is a North Germanic language that was spoken by inhabitants of Scandinavia and inhabitants of their overseas settlements during the Viking Age, until about 1300.

Old Norse was divided into three dialects: Old East Norse, Old West Norse, and Old Gutnish. Old West and East Norse formed a dialect continuum, with no clear geographical boundary between them. 

For example, Old East Norse traits were found in eastern Norway, although Old Norwegian is classified as Old West Norse, and Old West Norse traits were found in western Sweden. Most speakers spoke Old East Norse in what is present day Denmark and Sweden.

So from the Scandinavia whence the vikings derives from, being (Sweden, Iceland, Norway, and Denmark), Linguistic terms were formulated to create what was called Anglo Saxon (old English).