A fish is an animal. It is a multicellular organism with eukaryotic cells arranged into tissues and organs. Eukaryotic means that fish cells have nuclei and other structures that are protected by membranes.
Most fish reproduce sexually, can move around and have two full sets of their genetic material. Fish also eat other living organisms to get the energy necessary for life.
Fish are further specialized as animals that use gills to breathe, and don't have arms, legs, fingers or toes. They live almost exclusively in water and most are cold-blooded. Fish can have skeletons made of cartilage, such as in sharks, or made of bone, such as in tuna.