The word for fish

The word for fish in English and the other Germanic languages (German fisch; Gothic fisks) is inherited from Proto-Germanic, and is related to the Latin piscis and Old Irish īasc, though the exact root is unknown; some authorities reconstruct an Proto-Indo-European root *peysk-, attested only in Italic, Celtic, and Germanic.[8][9][10][11]

The English word once had a much broader usage than its current biological meaning. Names such as starfish, jellyfish, shellfish and cuttlefish attest to almost any fully aquatic animal (including whales) once being ‘fish’. “Correcting” such names (e.g. to ‘sea star’) is an attempt to retroactively apply the current meaning of ‘fish’ to words that were coined when it had a different meaning.

Evolution
Main article: Evolution of fish
Fish, as vertebrata, developed as sister of the tunicata. As the tetrapods emerged deep within the fishes group, as sister of the lungfish, characteristics of fish are typically shared by tetrapods, including having vertebrae and a cranium.

Drawing of animal with large mouth, long tail, very small dorsal fins, and pectoral fins that attach towards the bottom of the body, resembling lizard legs in scale and development.[12]
Dunkleosteus was a gigantic, 10-metre (33 ft) long prehistoric fish of class Placodermi.

Lower jaw of the placoderm Eastmanosteus pustulosus, showing the shearing structures (“teeth”) on its oral surface; from the Devonian of Wisconsin.
Early fish from the fossil record are represented by a group of small, jawless, armored fish known as ostracoderms. Jawless fish lineages are mostly extinct. An extant clade, the lampreys may approximate ancient pre-jawed fish. The first jaws are found in Placodermi fossils. They lacked distinct teeth, having instead the oral surfaces of their jaw plates modified to serve the various purposes of teeth. The diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors.

Fish may have evolved from a creature similar to a coral-like sea squirt, whose larvae resemble primitive fish in important ways. The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today), although perhaps the reverse is the case.