In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids. It is a quaternary structure of a protein.
A homo-dimer would be formed by two identical molecules (a process called homodimerization). A hetero-dimer would be formed by two different macromolecules (called heterodimerization).
Most dimers in biochemistry are not connected by covalent bonds. An example of a non-covalent heterodimer would be the enzyme reverse transcriptase, which is composed of two different amino acid chains. An exception is dimers that are linked by disulfide bridges such as the homodimeric protein NEMO.