one who, or that which, adds; esp., a machine for adding numbers.-a serpent.-a small venomous serpent of the genus vipera. the common european adder is the vipera (/ pelias) berus. the puff adders of africa are species of clotho.-in america, the term is commonly applied to several harmless snakes, as the milk adder, puffing adder, etc.-same as sea adder.