Consisting of or involving a single molecule: in chem., of a substance, having its simplest molecular composition in contradistinction from polymeric derivatives formed by the union of two, three, or more of the simplest molecules. Thus, formic aldehyde (CH2O) may be spoken of as monomolecular, and metaformaldehyde (C3H6O3) as its trimolecular derivative, 3CH2O.