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]