A fjord is a long, narrow, deep sea inlet with steep parallel walls, formed by the glacial deepening of a river valley over successive ice ages and subsequently drowned by rising seawater after the last glaciation. The English word is a direct loan from Old Norse fjǫrðr, and the Norwegian western coast is the type-site for the geological feature globally - geographers describe similar inlets in Chile, New Zealand, Greenland and British Columbia by the Norwegian word.
Norway's fjords are too many to count with any agreement, and six of them are longer than 100 kilometers. They run from the Oslofjord in the south-east to the Varangerfjord near the Russian border in the north, with the densest concentration along the western coast between Stavanger and Trondheim. The Sognefjord is the longest and deepest in the country, running 205 kilometers inland from the Atlantic and 1,303 meters deep at its deepest point. The Geirangerfjord and the Nærøyfjord (a side arm of the Sognefjord) became UNESCO World Heritage Sites in 2005.
The defining structural feature of a fjord is its depth. The water at the center of a major Norwegian fjord is typically deeper than the open sea outside - the Sognefjord at 1,303 meters deep is more than 1,000 meters deeper than the continental shelf it runs into. This unusual bathymetry produces the characteristic dark color of fjord water, the long sustained calm of the inner arms (sheltered from open-Atlantic swell), and the year-round liveability for cold-water marine species like the Norway lobster, the king crab and the Atlantic cod.
For travel, the main consequence is that a fjord is best experienced from the water rather than from above. Standing on a small-boat deck looking up at a 1,200-meter wall of granite is fundamentally different from looking down at the same wall from a viewpoint car park. We tend to route fjord trips through small electric ferries, sea kayaks and short RIB transfers rather than through the larger cruise vessels - see our long-form at The narrow fjords.