- (obsolete) A snake.
- A small venomous serpent of the genus Vipera. The common European adder is the Vipera (or Pelias) berus. The puff adders of Africa are species of the genus Clotho.
- (context, US, and, Canadian English) Any of several small nonvenomous snakes resembling the adder, such as the milk snake.
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Full Definition of adder