\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.