1.A combining form denoting in the middle, intermediate.
2.specif. (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.
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