The Collaborative International Dictionary of English v.0.48:

Sap \Sap\, n. [AS. saep; akin to OHG. saf, G. saft, Icel. safi;
   of uncertain origin; possibly akin to L. sapere to taste, to
   be wise, sapa must or new wine boiled thick. Cf. Sapid,
   Sapient.]
   1. The juice of plants of any kind, especially the ascending
      and descending juices or circulating fluid essential to
      nutrition.
      [1913 Webster]

   Note: The ascending is the crude sap, the assimilation of
         which takes place in the leaves, when it becomes the
         elaborated sap suited to the growth of the plant.
         [1913 Webster]

   2. The sapwood, or alburnum, of a tree.
      [1913 Webster]

   3. A simpleton; a saphead; a milksop. [Slang]
      [1913 Webster]

   Sap ball (Bot.), any large fungus of the genus Polyporus.
      See Polyporus.

   Sap green, a dull light green pigment prepared from the
      juice of the ripe berries of the Rhamnus catharticus, or
      buckthorn. It is used especially by water-color artists.


   Sap rot, the dry rot. See under Dry.

   Sap sucker (Zool.), any one of several species of small
      American woodpeckers of the genus Sphyrapicus,
      especially the yellow-bellied woodpecker (Sphyrapicus
      varius) of the Eastern United States. They are so named
      because they puncture the bark of trees and feed upon the
      sap. The name is loosely applied to other woodpeckers.

   Sap tube (Bot.), a vessel that conveys sap.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Sphyrapicus varius
    n 1: eastern North American sapsucker having a pale yellow
         abdomen [syn: yellow-bellied sapsucker, Sphyrapicus
         varius]