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
see also:
sapid sapient sap ball polyporus sap green rhamnus catharticus 
sap rot dry sap sucker sphyrapicus sphyrapicus varius 
sap tube