The Collaborative International Dictionary of English v.0.48:

Woodpecker \Wood"peck`er\, n. (Zool.)
   Any one of numerous species of scansorial birds belonging to
   Picus and many allied genera of the family Picidae.
   [1913 Webster]

   Note: These birds have the tail feathers pointed and rigid at
         the tip to aid in climbing, and a strong chisellike
         bill with which they are able to drill holes in the
         bark and wood of trees in search of insect larvae upon
         which most of the species feed. A few species feed
         partly upon the sap of trees (see Sap sucker, under
         Sap), others spend a portion of their time on the
         ground in search of ants and other insects.
         [1913 Webster] The most common European species are the
         greater spotted woodpecker (Dendrocopus major), the
         lesser spotted woodpecker (Dendrocopus minor), and
         the green woodpecker, or yaffle (see Yaffle).
         [1913 Webster] The best-known American species are the
         pileated woodpecker (see under Pileated), the
         ivory-billed woodpecker (Campephilus principalis),
         which is one of the largest known species, the
         red-headed woodpecker, or red-head (Melanerpes
         erythrocephalus), the red-bellied woodpecker
         (Melanerpes Carolinus) (see Chab), the superciliary
         woodpecker (Melanerpes superciliaris), the hairy
         woodpecker (Dryobates villosus), the downy woodpecker
         (Dryobates pubescens), the three-toed, woodpecker
         (Picoides Americanus), the golden-winged woodpecker
         (see Flicker), and the sap suckers. See also
         Carpintero.
         [1913 Webster]

   Woodpecker hornbill (Zool.), a black and white Asiatic
      hornbill (Buceros pica) which resembles a woodpecker in
      color.
      [1913 Webster]

WordNet (r) 3.0 (2006):

woodpecker
    n 1: bird with strong claws and a stiff tail adapted for
         climbing and a hard chisel-like bill for boring into wood
         for insects [syn: woodpecker, peckerwood, pecker]