Source : Webster's Revised Unabridged Dictionary (1913)

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

   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, whish was formerly
         supposed to have fallen from the sky, whence the
         popular names, fallen star and star jelly. Also called
         witches' butter.

Source : WordNet®

nostoc
     n : found in moist places as rounded jellylike colonies