The word Viking has come to be used to describe all early medieval Scandinavians, but as originally used by contemporaries the term vikingr only to someone who went i viking, that is plundering. Only a minority of early medieval Scandinavians were, therefore, Vikings in the strict sense of the word. Various explanations of the origin of the word have been proposed. The commonest explanations are that Viking is derived either from Viken in southern Norway, and therefore means simply 'the men from Viken', or that it comes from the Scandinavian word vik (bay or cove) and means 'the men from the bays'. Another possibility is that it derives from the old Scandinavian verb vikya, meaning 'to turn away' and so came to be used to describe Scandinavians who were travelling away from home. However, a Scandinavian origin for the word seems unlikely, as its use in Old English predates the Viking Age. It was used to describe any band of pirates, not simply those from Scandinavia. In the 8th century poem, Exodus, the seafaring sons of Reuben are described as wicingas, for example. In this case, it seems likely that wicing is derived from Old English wic (port of trade) and means 'the men who frequent (or attack) ports', and that it was only later adopted by the Scandinavians themselves. After falling into disuse in the central Middle Ages, the use of Viking was revived by the 19th century Romantic movement.
Notes from Encyclopaedia of the Viking Age by John Haywood. Thames & Hudson 2000. pp 198-199.