rheotannic,a.Chem.|riːəʊˈtænɪk| [f.Gr. ῥῆον rheum2 + tannic.] The name of a variety of tannic acid found in rhubarb. 1883Encycl.Brit. XX. 530/1 The chief chemical constituents of Chinese rhubarb are chrysophan (C15H10O4), rheotannic acid (C20H26O14) [etc.].