Meso- /(?)/

Mes·o-

Meso-
  1. A combining form denoting in the middle, intermediate.
  2. Denoting a type of hydrocarbons which are regarded as methenyl derivatives. Also used adjectively. (Chem.)
  3. 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. (Chem.)
  4. Attached to a carbon atom located between two hereoatoms in a heterocyclic ring.