Dogfish, also known as a cat shark, can have many meanings, but usually refers to a type of shark:
A shark belonging to the order Squaliformes, or to one of its constituent families:
Family Centrophoridae (gulper sharks)
Family Dalatiidae (kitefin sharks)
Family Etmopteridae (lantern sharks)
Family Oxynotidae (rough sharks)
Family Somniosidae (sleeper sharks)
Family Squalidae (dogfish sharks), and in particular the spiny dogfish (Squalus acanthias)
The Spiny dogfish is initially close in appearence to another shark, the Smooth dogfish, or, Smooth hound-shark (Family Triakidae), but can be easily distinguished. The Spiny version has a single spine in front of each of its two dorsal fins. The Spiny dogfish does not possess an anal fin, the Smooth houndshark does.