Fish are a paraphyletic group of organisms that consist of all gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

-bearing aquatic
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...

Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 (or craniate) animals that lack limbs
Limb (anatomy)
A limb is a jointed, or prehensile , appendage of the human or other animal body....

 with digits
Digit (anatomy)
A digit is one of several most distal parts of a limb, such as fingers or toes, present in many vertebrates.- Names:Some languages have different names for hand and foot digits ....

. Included in this definition are the living hagfish
Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...

, lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

s, and cartilaginous
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...

 and bony fish, as well as various extinct related groups. Most fish are ectotherm
An ectotherm, from the Greek εκτός "outside" and θερμός "hot", refers to organisms that control body temperature through external means. As a result, organisms are dependent on environmental heat sources and have relatively low metabolic rates. For example, many reptiles regulate their body...

ic ("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna
Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...

 can hold a higher core temperature.

Fish and guests in three days are stale.

John Lyly, Euphues (1579)

Fish not, with this melancholy bait,For this fool-gudgeon, this opinion.

William Shakespeare, The Merchant of Venice, Act I, sc. i

A man may fish with the worm that hath eat of a king, and eat of the fish that hath fed of that worm.

William Shakespeare, Hamlet, Act IV, sc. iii

Bait the hook well: this fish will bite.

William Shakespeare, Much Ado About Nothing, Act II, sc. iii

What have we here? a man or a fish? dead or alive? A fish: he smells like a fish; a very ancient and fish-like smell.

William Shakespeare, The Tempest, Act II, sc. ii

I am, out of the ladies' company, like a fish out of water.

Thomas Shadwell, A True Widow, Act III, sc. i (1679)

It was always the biggest fish I caught that got away.

Eugene Field, Our Biggest Fish, st. 2

Only the gamefish swims upstream,But the sensible fish swims down.

Ogden Nash, When You Say That, Smile

People would cry for the death of a bird, but never for the blood of a fish. Blessed are those who have a voice.

Ghost in the Shell