a kind of gum procured from a spiny leguminous shrub (astragalus gummifer) of western asia, and other species of astragalus. it comes in hard whitish or yellowish flakes or filaments, and is nearly insoluble in water, but slowly swells into a mucilaginous mass, which is used as a substitute for gum arabic in medicine and the arts. called also gum tragacanth.