The IUCN reported in 2008, 'The taxonomy of this genus is clearly in need of review, and it is likely that O. The three to five types of orcas may be distinct enough to be considered different races, subspecies, or possibly even species (see Species problem). In contrast, a 2019 phylogenetic study found the orca to be the second most basal member of the Delphinidae, with only the Atlantic white-sided dolphin ( Leucopleurus acutus) being more basal.

However, a more recent (2018) study places the orca as a sister taxon to the Lissodelphininae, a clade that includes Lagenorhynchus and Cephalorhynchus. Although it has morphological similarities with the false killer whale, the pygmy killer whale and the pilot whales, a study of cytochrome b gene sequences indicates that its closest extant relatives are the snubfin dolphins of the genus Orcaella. The orca lineage probably branched off shortly thereafter. The orca is one of 35 species in the oceanic dolphin family, which first appeared about 11 million years ago. Konrad Gessner wrote the first scientific description of an orca in his Piscium & aquatilium animantium natura of 1558, part of the larger Historia animalium, based on examination of a dead stranded animal in the Bay of Greifswald that had attracted a great deal of local interest. Orcinus orca is the only recognized extant species in the genus Orcinus, and one of many animal species originally described by Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae. This meaning of 'grampus' should not be confused with the genus Grampus, whose only member is Risso's dolphin. 'Grampus' is a former name for the species, but is now seldom used. They are sometimes referred to as 'blackfish', a name also used for other whale species.

As part of the family Delphinidae, the species is more closely related to other oceanic dolphins than to other whales. orcae) for these animals, possibly borrowing Ancient Greek ὄρυξ ( óryx), which referred (among other things) to a whale species. The genus name Orcinus means 'of the kingdom of the dead', or 'belonging to Orcus'. Since the 1960s, the use of 'orca' instead of 'killer whale' has steadily grown in common use. Orcas are commonly referred to as 'killer whales', despite being a type of dolphin. Orcas feature strongly in the mythologies of indigenous cultures, and their reputation in different cultures ranges from being the souls of humans to merciless killers. There have been cases of captive orcas killing or injuring their handlers at marine theme parks.

Wild orcas are not considered a threat to humans, and no fatal attack on humans has ever been documented. In late 2005, the southern resident orcas, which swim in British Columbia and Washington waters, were placed on the U.S. Some local populations are considered threatened or endangered due to prey depletion, habitat loss, pollution (by PCBs), capture for marine mammal parks, and conflicts with human fisheries. The International Union for Conservation of Nature assesses the orca's conservation status as data deficient because of the likelihood that two or more orca types are separate species. Their sophisticated hunting techniques and vocal behaviours, which are often specific to a particular group and passed across generations, have been described as manifestations of animal culture. They are highly social some populations are composed of very stable matrilineal family groups (pods) which are the most stable of any animal species. Orcas are apex predators, as they have no natural predators. They have been known to attack baleen whale calves, and even adult whales. Some feed exclusively on fish, while others hunt marine mammals such as seals and other species of dolphin. Orcas have a diverse diet, although individual populations often specialize in particular types of prey. A cosmopolitan species, orcas can be found in all of the world's oceans in a variety of marine environments, from Arctic and Antarctic regions to tropical seas. It is recognizable by its black-and-white patterned body. The orca or killer whale ( Orcinus orca) is a toothed whale belonging to the oceanic dolphin family, of which it is the largest member.

False killer whale