Fish a limbless cold-blooded vertebrate animal with gills and fins living wholly in water.
The earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed external armor that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators rather than just the prey of arthropods.
Now coming to the scientific defination of fish...
It is difficult to define what makes a fish a fish because there is so much diversity among animals that we consider to be fishes as there are more than 27,900 species of fishes alive today, living in marine and freshwaters. However all fishes …
* Have a brain protected by a braincase and an obvious head region with eyes, teeth, and other sensory organs
and Most fishes …
* are vertebrates with vertebrae protecting the spinal cord
* live in water
* breathe primarily with gills rather than lungs
* have paired limbs, in the form of fins that aid in locomotion
* are unable to regulate their own internal body temperatures
* are covered with scales that protect their bodies