The term billfish refers to a group of predatory fish characterised by prominent bills, or rostrums, and by their large size (some are longer than four metres (13 feet)). Billfish include sailfish and marlin, which make up the family Istiophoridae, and swordfish, sole member of the family Xiphiidae. They are apex predators which feed on a wide variety of smaller fish, crustaceans, and cephalopods.

Billfish are pelagic and highly migratory. They are found in all oceans, although they usually inhabit tropical and subtropical waters; swordfish are found in temperate waters as well. Billfish use their long spears or swordlike upper jaws or beaks to slash at and stun prey during feeding. Their bills can also be used to spear prey, and have been known to spear boats (probably accidentally), but they are not normally used in that way. They are highly valued as gamefish by sports fishermen.

The term billfish refers to the fishes of the families Xiphiidae and Istiophoridae. These large fishes are "characterized by the prolongation of the upper jaw, much beyond the lower jaw into a long rostrum which is flat and swordlike (swordfish) or rounded and spearlike (sailfishes, spearfishes and marlins)."