The Collaborative International Dictionary of English v.0.48:

Aster \As"ter\ ([a^]s"t[~e]r), n. [L. aster aster, star, Gr.
   'asth`r star. See Star.]
   1. (Bot.) A genus of herbs with compound white or bluish
      flowers; starwort; Michaelmas daisy.
      [1913 Webster]

   2. (Floriculture) A plant of the genus Callistephus. Many
      varieties (called China asters, German asters, etc.)
      are cultivated for their handsome compound flowers.
      [1913 Webster]

   3. (Biol.) A star-shaped figure of achromatic substance found
      chiefly in cells dividing by mitosis.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

aster
    n 1: any of various chiefly fall-blooming herbs of the genus
         Aster with showy daisylike flowers
    2: star-shaped structure formed in the cytoplasm of a cell
       having fibers like rays that surround the centrosome during
       mitosis