Flying fish is a true fish as per the biological system of classification. Flying fish, any of about 40 species of oceanic fishes of the family Exocoetidae (order Atheriniformes), found worldwide in warm waters and are noted for their ability to fly. They are all small, attaining a maximum length of about 45 cm (18 inches), and have wing like, rigid fins and an unevenly forked tail. Silverfish are not fish, they're insects. The name silverfish is derived from their silvery grey and blue colouring along with the fish like appearance of its movements. Jellyfish are not true fish, they are vertebrates.