A venomous snake (family Viperidae) that has a stocky body, a broad, triangular head, ridged scales, and a short tail. A viper has two fangs at the front of its mouth that fold back into grooves in the roof of the mouth when not in use, but are erected, with the mouth open at almost 180°, when the snake strikes. Vipers can control the amount of venom they inject, and when striking in self-defence may inject none. There are about 224 species occurring worldwide except in Antarctica, the Arctic, Australia, New Zealand, Hawaii, Madagascar, and some Pacific islands.