D-dimer: A fragment produced during the degradation of a clot. The D here stands for domain. Dimer indicates two identical units, in this case two identical domains. D-dimer result from complete breakdown of the clot. Monoclonal antibody to the D-dimer fragment provide the basis for the main methods of detecting it. The presence of D-dimers in the blood is a reliable clue that clotting has begun. Sometimes written d-dimer or D-Dimer. Pronounced deemer.