A genus of algæ. The plants are composed of moniliform cells imbedded in a gelatinous substance. [1913 Webster
" 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."