The Collaborative International Dictionary of English v.0.48:

Nostoc \Nos"toc\, prop. n. [F.] (Bot.)
   A genus of algae. The plants are composed of moniliform cells
   imbedded in a gelatinous substance.
   [1913 Webster]

   Note: Nostoc commune is found on the ground, and is
         ordinarily not seen; but after a rain it swells up into
         a conspicuous jellylike mass, which was formerly
         supposed to have fallen from the sky, whence the
         popular names, fallen star and star jelly. Also
         called witches' butter.
         [1913 Webster]

WordNet (r) 3.0 (2006):

nostoc
    n 1: found in moist places as rounded jellylike colonies