The Collaborative International Dictionary of English v.0.48:

Tulip \Tu"lip\ (t[=u]"l[i^]p), n. [F. tulipe, OF. also tulipan,
   It. tulipano, tulipa, from Turk. tulbend, dulbend, literally,
   a turban, Per. dulband; -- so called from the resemblance of
   the form of this flower to a turban. See Turban.] (Bot.)
   Any plant of the liliaceous genus Tulipa. Many varieties
   are cultivated for their beautiful, often variegated flowers.
   [1913 Webster]

   Tulip tree.
   (a) A large American tree (Liriodendron tulipifera) of the
       magnolia family (Magnoliaceae) bearing tuliplike
       flowers. See Liriodendron.
   (b) A West Indian malvaceous tree (Paritium tiliaceum syn.
       Hibiscus tiliaceum).
       [1913 Webster + PJC]

WordNet (r) 3.0 (2006):

tulip
    n 1: any of numerous perennial bulbous herbs having linear or
         broadly lanceolate leaves and usually a single showy flower