- Noun. (obsolete) A snake.
- Noun. (British) 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''.
- Noun. (US, Canada) Any of several small nonvenomous snakes resembling the adder, such as the milk snake.
- Noun. Someone who or something which performs arithmetic addition.
- Noun. Something which adds or increases.