A kind of dimer, or something made up of two parts, in which the two parts are not identical.
Occurs quite frequently in nature, as a number of proteins, nucleic acids, lipids, and polysaccharides are composed of two or more non-identical components.
See also homodimer, oligomer, polymer, monomer. The prefix hetero can be used with oligomer and polymer to denote whether their constituents are identical or not.