Orcas are dolphins they are called killer whales because of their size and how they kill. They kill use mostly their weight and the strength of their jaws.
An orca, or killer whale, is in the family Delphinidae, making it a dolphin by classification. However, all dolphins are in the order Cetacea, making all dolphins to be whales.

Dolphins (and thus orcas) a family in the sub-order of toothed whales, which belongs to the order of whales.

So an orca is both a dolphin and a whale, albeit "dolphin" is a more specific classification.
The Orca is actually categorized as a dolphin. This is because dolphins are a sub-category of a sub-category of whales. This makes all dolphins whales, but it is necessary to note that not all whales are dolphins. ( Like the "All squares are rectangles, but not all rectangles are squares" theory ) The Orca, or Killer whale as it was errantly named, is in fact, a dolphin. It is the largest of the dolphins. Links to the relevant Wikipedia articles are provided below.

Scientifically, the Orca is a dolphin. It is a member of the family of whales called delphinidae, the dolphin family. In this family you will find the bottle-nose, common, and Hector's dolphins. Take a look at the shape of the Hector's dolphin and you'll see that the Orca looks more like it than any of the larger species of whale.

Scientifically, the term whale includes dolphins and porpoises. These all fall under the order of mammals called Cetacea. The order is divided into the two sub-orders Mysticeti, or baleen whales, and Odontoceti, or toothed whales. Also, there is an extinct order called Archaeoceti. Odontoceti includes ten families, one of which is Delphinidae, under which Orcinus Orca, or Killer whale, falls.

Common usage of the word dolphin however, only includes the smaller variety of dolphins leaving out the Orca, Pilot Whales, and a few other species.

The Killer Whale is the largest of the dolphin family, renowned as an apex predator.

It is interesting to note that they are in fact Whale Killers, often observed drowning the calf of a larger species of whale after separating it from its' pod.

Killer Whales are Dolphins and they belong to the family delphidane. They are the biggest Dolphins, but have been known to attack members of their own type.

They are classified as toothed whales belonging to the oceanic dolphin family. They are highly successful at preying upon other, larger whale species. Orcas are the only animal other than humans to prey upon Polar bears.

See more at the related link(s), listed below:
It is a member of Dolphin family (Delphinidae),which is a part of the bigger Whale Species group. So,every Dolphin(including Killer Whales,or Orcas) are necessarily a part of Whale species (but the reverse is not true)

In common use though - we can define an Orca as a Dolphin, not a Whale.
dolphins
Orcas are classified in the Oceanic Dolphin Family, so I would say that the are dolphins, but all dolphins are small whales, so they could be both.
Killer whales are actually dolphins. They are the largest member of the dolphin family and are commonly referred to as orcas.
A Killer Whale, or Orca, is the largest of the true dolphins, family Delphinidae. That is probably the only reason it is called a whale instead of a dolphin. In addition to the Delphinidae, just about any and every small cetacean has been called a dolphin, and all of the largest of the dolphins, killer whales and pilot whales, are all called whales so both terms are mostly about size, and neither is specific.
Killer whales are whales

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