A compound or unit produced by the combination of two like molecules; in the strictest sense, this occurs without the loss of atoms (thus, nitrogen tetroxide, N2O4, is the dimer of nitrogen dioxide, NO2), but usually by elimination of H2O or a similar small molecule between the two (e.g., a disaccharide), or by simple noncovalent association (as of two identical protein molecules); higher orders of complexity are called trimers, tetramers, oligomers, and polymers. [G. di-, two, + -mer]