What “Viking” means
The Old Norse word víkingr meant a raider or pirate — someone going on a víking, a raiding expedition. The term in modern usage covers the broader Scandinavian seafaring culture of the period, including raiders, traders, settlers, mercenaries, and explorers. Contemporary western European sources called them Northmen, Danes (regardless of actual origin), or pagans.
Why and when
The conventional start of the Viking Age is the raid on the English monastery of Lindisfarne on 8 June 793. The raid was not strictly the first Viking attack on Christian Europe, but it produced the most dramatic surviving documentary account and is conventionally treated as the symbolic opening.
The underlying causes are debated. Probable factors include the development of efficient long-distance Scandinavian seacraft (the clinker-built longship became operational in the 8th century), demographic pressure in the Scandinavian homeland, the relative wealth of unguarded coastal Christian monasteries, and possibly climate-related agricultural changes during the early Medieval Warm Period.
What they did
Viking activity over the following 270 years took several distinct forms:
Raiding. Coastal monasteries, towns, and even substantial cities (Hamburg in 845, Paris in 845 and 885, Cordoba in 844) were attacked. Substantial portions of Britain, Ireland, and Francia were repeatedly raided. The political weakness of the post-Charlemagne Carolingian successors made Francia particularly vulnerable.
Trading. Vikings traded along the major European rivers and across the Baltic. The Eastern Vikings (Swedes, mostly) descended the Russian rivers to reach the Black Sea and Constantinople, founding the principalities of Novgorod and Kyiv (the precursors of modern Russia and Ukraine). The Old Norse word Rus — applied to these Eastern Vikings — is the etymological root of the word Russia.
Settlement. Vikings established permanent communities in northern and eastern England (the Danelaw), in Ireland (Dublin, Wexford, Limerick, Cork were Viking foundations), in Normandy (granted by treaty to the Norwegian-Danish leader Rollo in 911 — the descendants of these settlers would conquer England in 1066), in the Hebrides, the Orkneys, the Shetlands, and the Faroes. They settled Iceland from c. 870 and Greenland from 985 under Erik the Red.
Exploration. Leif Erikson sailed from Greenland to the eastern coast of North America around 1000, establishing a short-lived settlement at L’Anse aux Meadows on the northern tip of Newfoundland. The site was rediscovered and confirmed by archaeology in the 1960s. Norse contact with North America preceded Columbus by approximately five centuries.
How it ended
The Viking Age conventionally ends at the Battle of Stamford Bridge on 25 September 1066, in which the English king Harold Godwinson defeated and killed the invading Norwegian king Harald Hardrada. Three weeks later Harold himself was killed at Hastings by the Norman duke William the Conqueror — whose own ancestry was Viking. The Norman conquest of England was, in a real sense, a Viking conquest by descendants of Vikings who had been settled in France for a hundred and fifty years.
The transition from “Viking” to “medieval Scandinavian” was gradual. The Norse kingdoms (Norway, Denmark, Sweden) had converted to Christianity by the late 11th century and adopted European feudal political forms. The seafaring tradition continued: Norse Greenland survived until approximately 1450 before climate change finished it and Hans Egede arrived three centuries later to find the ruins.
Legacy
The Scandinavian languages, the th and gh sounds in English, hundreds of English place names ending in -by and -thorpe, the political traditions of medieval Iceland (the Althing, often called the oldest continuous parliament in the world), the Cyrillic alphabet’s transmission to Russia, the foundation of Normandy, the genetic profile of much of the British Isles, and a substantial body of saga literature all descend from the Viking Age. The Old Norse Eddas and Sagas, written down in Iceland from the 12th to 14th centuries, are among the most substantial bodies of vernacular medieval literature in any European language.