Of a part of a plant: consisting of one element. Later also (of the thallus of a coralline alga): undifferentiated.
Early 19th century (in an earlier sense). From mono- + -merous, after French monomère. Compare Hellenistic Greek μονομερής (of philosophy) consisting of one part (Sextus Empiricus), (of a kind of bandage) for one side (Galen) from ancient Greek μονο- + μέρος part.