The Collaborative International Dictionary of English v.0.48:

Rowan tree \Row"an tree`\ [Cf. Sw. r["o]nn, Dan. r["o]nne, Icel.
   reynir, and L. ornus.] (Bot.)
   A european tree (Pyrus aucuparia) related to the apple, but
   with pinnate leaves and flat corymbs of small white flowers
   followed by little bright red berries. Called also roan
   tree, and mountain ash. The name is also applied to two
   American trees of similar habit (Pyrus Americana, and
   Pyrus sambucifolia).
   [1913 Webster]

WordNet (r) 3.0 (2006):

rowan tree
    n 1: Eurasian tree with orange-red berrylike fruits [syn:
         rowan, rowan tree, European mountain ash, Sorbus
         aucuparia]