Meso- \Mes"o-\, Mes- \Mes-\ [Gr. me`sos in the middle.]
   1. A combining form denoting in the middle, intermediate.
      [1913 Webster]

   2. specif. (Chem.), Denoting a type of hydrocarbons which are
      regarded as methenyl derivatives. Also used adjectively.
      [1913 Webster]

   3. (Chem.) Optically inactive due to compensating optical
      activity at two or more asymmetric centers, often due to a
      plane of symmetry within the molecular structure; as,
      meso-tartaric acid.
      [PJC]

   4. Attached to a carbon atom located between two hereoatoms
      in a heterocyclic ring.
      [PJC]