- (countable) An adder is a short, rather massive poisonous snake with long front teeth that poison its victims by biting them.
- Don't walk in the bush unprotected, you may be bitten by an adder!
- Adder can also stand for any poisonous snake.
- Certain poisonous snakes in North America are called adders though they are not true adders.
- Someone or something that adds. Usually a part (circuit) of computer chip.
- The CPU contained extra adders to increase speed.