The gray whale is distributed in an eastern North Pacific (North American) population and a critically endangered western North Pacific (Asian) population. North Atlantic populations were extirpated (perhaps by whaling) on the European coast before AD 500 and on the American coast around the late 17th to early 18th centuries. However, on May 8, 2010, a sighting of a gray whale was confirmed off the coast of Israel in the Mediterranean Sea, leading some scientists to think they might be repopulating old breeding grounds that have not been used for centuries. In May and June 2013 a gray whale was sighted off the coast of Namibia – the first confirmed in the Southern Hemisphere. The round-trip journey of one gray whale has set a new record for the longest mammal migration, covering a distance of more than 22,000 kilometres across the Pacific Ocean. Her migration has shown new insight into how endangered species are making drastic changes in their life style.
The gray whale is traditionally placed as the only living species in its genus and family. Some recent DNA analyses have suggested that certain rorquals of the family Balaenopteridae, such as the humpback whale, Megaptera novaeangliae, and fin whale, Balaenoptera physalus, are more closely related to the gray whale than they are to some other rorquals, such as the minke whales. But other recent studies place gray whales as being outside the rorqual clade, but as the closest relatives to the rorquals.
John Edward Gray placed it in its own genus in 1865, naming it in honour of physician and zoologist Daniel Frederik Eschricht. The common name of the whale comes from its coloration. The subfossil remains of now extinct gray whales from the Atlantic coasts of England and Sweden were used by Gray to make the first scientific description of a species then surviving only in Pacific waters. The living Pacific species was described by Cope as Rhachianectes glaucus in 1869. Skeletal comparisons showed the Pacific species to be identical to the Atlantic remains in the 1930s, and Gray's naming has been generally accepted since. Although identity between the Atlantic and Pacific populations cannot be proven by anatomical data, its skeleton is distinctive and easy to distinguish from that of all other living whales.
Many other names have been ascribed to the gray whale, including desert whale, devilfish, gray back, mussel digger and rip sack. The name Eschrichtius gibbosus is sometimes seen; this is dependent on the acceptance of a 1777 description by Erxleben.
The gray whale has a dark slate-gray color and is covered by characteristic gray-white patterns, scars left by parasites which drop off in its cold feeding grounds. Individual whales are typically identified using photographs of their dorsal surface and matching the scars and patches associated with parasites that have fallen off the whale or are still attached. They have two blowholes on top of their head, which can create a distinctive heart-shaped blow at the surface in calm wind conditions.
Gray whales measure from 4.9 m (16 ft) in length for newborns to 13–15 m (43–49 ft) for adults (females tend to be slightly larger than adult males). Newborns are a darker gray to black in color. A mature gray whale can reach 40 t (44 short tons), with a typical range of 15–33 t (17–36 short tons).
Notable features that distinguish the gray whale from other mysticetes include its baleen that is variously described as cream, off-white, or blond in color and is unusually short. Small depressions on the upper jaw each contain a lone stiff hair, but are only visible on close inspection. Its head's ventral surface lacks the numerous prominent furrows of the related rorquals, instead bearing two to five shallow furrows on the throat's underside. The gray whale also lacks a dorsal fin, instead bearing 6 to 12 dorsal crenulations ("knuckles"), which are raised bumps on the midline of its rear quarter, leading to the flukes. This is known as the dorsal ridge. The tail itself is 3–3.5 m (10–11 ft) across and deeply notched at the center while its edges taper to a point.