A fish is any member of a paraphyletic group of organisms that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups. Most fish are ectothermic ("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature. Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., gulpers and anglerfish). With 33,100 described species, fish exhibit greater species diversity than any other group of vertebrates.

Fish are vertebrates with gills that live in water. Fish (food) is an important source of protein for humans and other animals.

Fish! is a text adventure game by Magnetic Scrolls released in . The game was designed by John Molloy, Phil South and Peter Kemp with contributions by Rob Steggles.

The Fish! Philosophy (styled FISH! Philosophy), modeled after the Pike Place Fish Market, is a technique to make individuals alert and active in the workplace. John Christensen created this philosophy in 1998 to improve "organizational culture". Its central four ideas, which have been adopted at some companies and schools, are:

1. choosing one’s attitude,
2. playing at work,
3. making someone’s day, and
4. being present.