The Collaborative International Dictionary of English v.0.48:

Adder \Add"er\, n. [See Add.]
   One who, or that which, adds; esp., a machine for adding
   numbers.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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. )
      [1913 Webster]

   2. (Zool.)
      (a) A small venomous serpent of the genus Vipera. The
          common European adder is the Vipera berus 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.
          [1913 Webster]

   Note: In the sculptures the appellation is given to several
         venomous serpents, -- sometimes to the horned viper
         (Cerastles).
         [1913 Webster]

WordNet (r) 3.0 (2006):

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

Easton's 1897 Bible Dictionary:

Adder
   (Ps. 140:3; Rom. 3:13, "asp") is the rendering of, (1.) Akshub
   ("coiling" or "lying in wait"), properly an asp or viper, found
   only in this passage. (2.) Pethen ("twisting"), a viper or
   venomous serpent identified with the cobra (Naja haje) (Ps.
   58:4; 91:13); elsewhere "asp." (3.) Tziphoni ("hissing") (Prov.
   23:32); elsewhere rendered "cockatrice," Isa. 11:8; 14:29; 59:5;
   Jer. 8:17, as it is here in the margin of the Authorized
   Version. The Revised Version has "basilisk." This may have been
   the yellow viper, the Daboia xanthina, the largest and most
   dangerous of the vipers of Palestine. (4.) Shephiphon
   ("creeping"), occurring only in Gen. 49:17, the small speckled
   venomous snake, the "horned snake," or cerastes. Dan is compared
   to this serpent, which springs from its hiding-place on the
   passer-by.


The Devil's Dictionary (1881-1906):

ADDER, n.  A species of snake.  So called from its habit of adding
funeral outlays to the other expenses of living.