The Collaborative International Dictionary of English v.0.48:

viper \vi"per\ (v[imac]"p[~e]r), n. [F. vip[`e]re, L. vipera,
   probably contr. fr. vivipera; vivus alive + parere to bring
   forth, because it was believed to be the only serpent that
   brings forth living young. Cf. Quick, a., Parent,
   Viviparous, Wivern, Weever.]
   [1913 Webster]
   1. (Zool.) Any one of numerous species of Old World venomous
      snakes belonging to Vipera, Clotho, Daboia, and
      other genera of the family Viperidae.
      [1913 Webster]

            There came a viper out of the heat, and fastened on
            his hand.                             --Acts xxviii.
                                                  3.
      [1913 Webster]

   Note: Among the best-known species are the European adder
         (Pelias berus), the European asp (Vipera aspis),
         the African horned viper (Vipera cerastes), and the
         Indian viper (Daboia Russellii).
         [1913 Webster]

   2. A dangerous, treacherous, or malignant person.
      [1913 Webster]

            Who committed
            To such a viper his most sacred trust
            Of secrecy.                           --Milton.
      [1913 Webster]

   3. Loosely, any venomous or presumed venomous snake.
      [PJC]

   Horned viper. (Zool.) See Cerastes.

   Red viper (Zool.), the copperhead.

   Viper fish (Zool.), a small, slender, phosphorescent
      deep-sea fish (Chauliodus Sloanii). It has long ventral
      and dorsal fins, a large mouth, and very long, sharp
      teeth.

   Viper's bugloss (Bot.), a rough-leaved biennial herb
      (Echium vulgare) having showy purplish blue flowers. It
      is sometimes cultivated, but has become a pestilent weed
      in fields from New York to Virginia. Also called blue
      weed.

   Viper's grass (Bot.), a perennial composite herb
      (Scorzonera Hispanica) with narrow, entire leaves, and
      solitary heads of yellow flowers. The long, white,
      carrot-shaped roots are used for food in Spain and some
      other countries. Called also viper grass.
      [1913 Webster]

WordNet (r) 3.0 (2006):

viper
    n 1: venomous Old World snakes characterized by hollow venom-
         conducting fangs in the upper jaw

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "viper":
   animal, beast, cur, dog, hound, hyena, insect, mongrel, ophidian,
   pig, pit viper, polecat, reptile, sea snake, serpent, skunk, snake,
   swine, varmint, vermin, whelp, worm



Easton's 1897 Bible Dictionary:

Viper
   In Job 20:16, Isa. 30:6; 59:5, the Heb. word eph'eh is thus
   rendered. The Hebrew word, however, probably denotes a species
   of poisonous serpents known by the Arabic name of 'el ephah.
   Tristram has identified it with the sand viper, a species of
   small size common in sandy regions, and frequently found under
   stones by the shores of the Dead Sea. It is rapid in its
   movements, and highly poisonous. In the New Testament _echidne_
   is used (Matt. 3:7; 12:34; 23:33) for any poisonous snake. The
   viper mentioned in Acts 28:3 was probably the vipera aspis, or
   the Mediterranean viper. (See ADDER.)