The Collaborative International Dictionary of English v.0.48:

Asterism \As`ter*ism\, n. [Gr. ?, fr. 'asth`r star; cf. F.
   ast['e]risme.]
   1. (Astron.)
      (a) A constellation. [Obs.]
      (b) A small cluster of stars.
          [1913 Webster]

   2. (Printing)
      (a) An asterisk, or mark of reference. [R.]
      (b) Three asterisks placed in this manner, [asterism], to
          direct attention to a particular passage.
          [1913 Webster]

   3. (Crystallog.) An optical property of some crystals which
      exhibit a star-shaped by reflected light, as star
      sapphire, or by transmitted light, as some mica.
      [1913 Webster]

WordNet (r) 3.0 (2006):

asterism
    n 1: (mineralogy) a star-shaped figure with six rays that is
         seen in some crystal structures under reflected or
         transmitted light
    2: (astronomy) a cluster of stars (or a small constellation)