The Collaborative International Dictionary of English v.0.48:

Grass tree \Grass" tree"\ (Bot.)
   (a) An Australian plant of the genus Xanthorrh[oe]a, having
       a thick trunk crowned with a dense tuft of pendulous,
       grasslike leaves, from the center of which arises a long
       stem, bearing at its summit a dense flower spike looking
       somewhat like a large cat-tail. These plants are often
       called "blackboys" from the large trunks denuded and
       blackened by fire. They yield two kinds of fragrant
       resin, called Botany-bay gum, and Gum Acaroides.
   (b) A similar Australian plant (Kingia australis).
       [1913 Webster]

WordNet (r) 3.0 (2006):

grass tree
    n 1: elegant tree having either a single trunk or a branching
         trunk each with terminal clusters of long narrow leaves and
         large panicles of fragrant white, yellow or red flowers;
         New Zealand [syn: cabbage tree, grass tree, Cordyline
         australis]
    2: any of several Australian evergreen perennials having short
       thick woody stems crowned by a tuft of grasslike foliage and
       yielding acaroid resins [syn: grass tree, Australian grass
       tree]
    3: gaunt Tasmanian evergreen shrubby tree with slender tapering
       leaves 3 to 5 feet long [syn: tree heath, grass tree,
       Richea pandanifolia]