By definition, fish are animals that live in water, have gills and lack limbs. They are also chordates, which means they have a backbone-like structure that provides support for the body and acts as an anchor for the major muscles.
Animals that live in water but have limbs rather than fins are not fish, but rather amphibians or reptiles. Most fish are cold-blooded, but there are a few exceptions to this generalization, including tuna, swordfish and certain types of sharks. Some animals that have the word "fish" in their name are not actually fish. For example, jellyfish are invertebrates, not chordates, and do not meet the definition of a fish.