The Collaborative International Dictionary of English v.0.48:

Gentian \Gen"tian\ (j[e^]n"shan or j[e^]n"sh[i^]*an), n. [OE.
   genciane, F. gentiane, L. gentiana, fr. Gentius, an Illyrian
   king, said to have discovered its properties.] (Bot.)
   Any one of a genus (Gentiana) of herbaceous plants with
   opposite leaves and a tubular four- or five-lobed corolla,
   usually blue, but sometimes white, yellow, or red. See
   Illust. of Capsule.
   [1913 Webster]

   Note: Many species are found on the highest mountains of
         Europe, Asia, and America, and some are prized for
         their beauty, as the Alpine (Gentiana verna,
         Gentiana Bavarica, and Gentiana excisa), and the
         American fringed gentians (Gentiana crinita and
         Gentiana detonsa). Several are used as tonics,
         especially the bitter roots of Gentiana lutea, the
         officinal gentian of the pharmacopoeias.
         [1913 Webster]

   Horse gentian, fever root.

   Yellow gentian (Bot.), the officinal gentian (Gentiana
      lutea). See Bitterwort.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gentian
    n 1: any of various plants of the family Gentianaceae especially
         the genera Gentiana and Gentianella and Gentianopsis