From 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.]
From 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