noun a serpent.
2. adder ·noun ·same·as sea adder.
3. adder ·noun one who, or that which, adds; ·esp., a machine for adding numbers.
4. adder ·noun in america, the term is commonly applied to several harmless snakes, as the milk adder, puffing adder, ·etc.
5. adder ·noun 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.