- 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.