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