Fish are aquatic vertebrates that are typically cold-blooded, covered with scales, and equipped with two sets of paired fins and several unpaired fins. Fish are abundant in the sea and in fresh water, with species being known from mountain streams (e.g., char and gudgeon); as well as in the deepest depths of the ocean (e.g., gulpers and anglerfish);.

They are of tremendous importance as food for people around the world, either collected from the wild (see fishing); or farmed in much the same way as cattle or chickens (see aquaculture);. Fish are also exploited for recreation, through angling and fishkeeping, and fish are commonly exhibited in public aquaria. Fish have an important role in many cultures through the ages, ranging as wide as deities and religious symbols to subjects of books and popular movies.

The term "fish" is most precisely used to describe any non-tetrapod chordate, i.e., an animal with a backbone, has gills throughout life, and have limbs, if any, in the shape of fins. Unlike groupings such as birds or mammals, fish are not a single clade but a paraphyletic collection of taxa, including hagfishes, lampreys, sharks and rays, ray-finned fishes, coelacanths, and lungfishes.

A typical fish is cold-blooded; has a streamlined body that allows it to swim rapidly; extracts oxygen from the water using gills; has two sets of paired fins, one or two dorsal fins, an anal fin, and a tail fin; has jaws; has skin that is usually covered with scales; and lays eggs that are fertilised externally.