Source : Webster's Revised Unabridged Dictionary (1913)

Adder \Add"er\, n. [See {Add}.]
   One who, or that which, adds; esp., a machine for adding
   numbers.

Adder \Ad"der\, n. [OE. addere, naddere, eddre, AS. n[ae]dre,
   adder, snake; akin to OS. nadra, OHG. natra, natara, Ger.
   natter, Goth. nadrs, Icel. na[eth]r, masc., na[eth]ra, fem.:
   cf. W. neidr, Gorn. naddyr, Ir. nathair, L. natrix, water
   snake. An adder is for a nadder.]
   1. A serpent. [Obs.] ``The eddre seide to the woman.''
      --Wyclif. Gen. iii. 4. )

   2. (Zo["o]l.)
      (a) 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
          {Clotho}.
      (b) In America, the term is commonly applied to several
          harmless snakes, as the {milk adder}, {puffing adder},
          etc.
      (c) Same as {Sea Adder}.

   Note: In the sculptures the appellation is given to several
         venomous serpents, -- sometimes to the horned viper
         ({Cerastles}).

Source : WordNet®

adder
     n 1: a person who adds numbers
     2: a machine that adds numbers
     3: small terrestrial viper common in northern Eurasia [syn: {common
        viper}, {Vipera berus}]