noun

• (obsolete) Any snake.
  "CALIBAN: His spirits hear me, / And yet I needs must curse. But they'll nor pinch / Fright me with urchin-shows, pitch me i'th' mire, / Nor lead me like a firebrand in the dark / Out of my way, unless he bid 'em; but / For every trifle are they set upon me, / Sometimes like apes that mow and chatter at me, / And after bite me; then like hedgehogs, which / Lie tumbling in my barefoot way, and mount / Their pricks at my footfall; sometimes am I / All wound with adders, who with their cloven tongues / Do hiss me into madness—"
• A name loosely applied to various snakes more or less resembling a viper.
  "Entirely filled with the image of another, her heart, indeed, had the deaf ear of the adder, which heedeth not the voice of the charmer, charm he never so wisely."
• A name loosely applied to various snakes more or less resembling a viper.
• A name loosely applied to various snakes more or less resembling a viper.

noun

• (informal) A person who has attention deficit disorder.
  "Many ADDers become targets for bullies and are routinely harassed at school."