The Collaborative International Dictionary of English v.0.48:

Xylem \Xy"lem\, n. [Gr. xy`lon wood.] (Bot.)
   That portion of a fibrovascular bundle which has developed,
   or will develop, into wood cells; -- distinguished from
   phloem.
   [1913 Webster]

WordNet (r) 3.0 (2006):

xylem
    n 1: the woody part of plants: the supporting and water-
         conducting tissue, consisting primarily of tracheids and
         vessels