mes-

 A combining form denoting in the middle, intermediate.  [1913 Webster]

 Denoting a type of hydrocarbons which are regarded as methenyl derivatives. Also used adjectively.  [1913 Webster]

 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]

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