What are the unique characteristics of fish?

I believe the question relates to the group of animals within the animal kingdom which is labeled as fish.

Fish are back boned animals that live in water.  The various kinds of fish differ so much in their shape, colour, and size that they do not seem to have much in common to be classified in a common group of animals.

However, all fish have two main features in common. First hey have a backbone, and so they are vertebrates. Second, they breathe mainly by means of gills.  Nearly all fish being cold-blooded animals, cannot regulate their body temperature, which changes with the temperature of their surroundings.  Also, almost all fish have fins, which are used for swimming.

Not all animals living in water are classified as fish. They differ from fish in at least one of these ways. For example, Dolphins, look like fish and have a backbone and fins, but they are breathe with lungs rather than gills, and they are warm-blooded. Some water animals such as jellyfish and starfish are called fish, but they do not have a backbone and so are not fish.