The fin whale (Balaenoptera physalus), also called the finback whale, razorback, or common rorqual, is a marine mammal belonging to the suborder of baleen whales. It is the second longest animal in the world and second largest rorqual after the blue whale, growing to over 27 metres (89 ft) long and weighing nearly 74 tonnes (73 long tons; 82 short tons). The American naturalist Roy Chapman Andrews called the fin whale "the greyhound of the sea... for its beautiful, slender body is built like a racing yacht and the animal can surpass the speed of the fastest ocean steamship."

Long and slender, the fin whale's body is brownish-grey with a paler underside. There are at least two recognized subspecies: the fin whale of the North Atlantic, and the fin whale of the Southern Hemisphere. It is found in all the world's major oceans, from polar to tropical waters. It is absent only from waters close to the ice pack at both the north and south poles and relatively small areas of water away from the open ocean. The highest population density occurs in temperate and cool waters. Its food consists of small schooling fish, squid, and crustaceans including copepods and krill.

A fin is a surface used for stability and/or to produce lift and thrust or to steer while traveling in water, air, or other fluid media, (in other words, a foil (fluid mechanics)). The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man-made devices. Fins, as with other foils, operate in fluids such as water or air.

Fins are seen both in nature and in manmade iterations.

Swimming water animals such as fish and cetaceans actively use pectoral fins for maneuvering, and dorsal fins contribute stability as the animal swims, propelling and maneuvering with its tail, itself recognizable as a fin.

The fin on fixed-wing aircraft is known as a vertical stabilizer. Fins are also seen used as e.g., fletching on arrows and at the rear of some bombs, missiles, rockets, and self-propelled torpedoes. These are typically "planar" (shaped like small wings), although grid fins are sometimes used in specialized cases.

Examples of fins include:

In surfing, there are two major types of (typically stationary) surfboard fins (hydrofoils), and a host of illustrative issues.

Whale (origin Old English hwæl) is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti (toothed whales). This suborder also includes the sperm whale, killer whale, pilot whale, and beluga whale. The other Cetacean suborder Mysticeti (baleen whales) are filter feeders that eat small organisms caught by straining seawater through a comblike structure found in the mouth called baleen. This suborder includes the blue whale, the humpback whale, the bowhead whale and the minke whale. All Cetacea have forelimbs modified as fins, a tail with horizontal flukes, and nasal openings (blowholes) on top of the head.

Whales range in size from the blue whale, the largest animal known ever to have existed at 30 m (98 ft) and 180 tonnes (180 long tons; 200 short tons), to various pygmy species, such as the pygmy sperm whale at 3.5 m (11 ft).

Whales collectively inhabit all the world's oceans and number in the millions, with annual population growth rate estimates for various species ranging from 3% to 13%. For centuries, whales have been hunted for meat and as a source of raw materials. By the middle of the 20th century, however, industrial whaling had left many species seriously endangered, leading to the end of whaling in all but a few countries.