any of various plants some parts of which can be used as soap; esp., a tall W North American plant (Chlorogalum pomeridianum) of the lily family
See soap plant in American Heritage Dictionary 4
noun
Any of several bulbous plants of the genus Chlorogalum of western North America, having small white or purple flowers. The edible bulbs of several species, especially C. pomeridianum, produce a soapy lather when crushed.