The Collaborative International Dictionary of English v.0.48:
Meso- \Mes"o-\, Mes- \Mes-\ [Gr. me`sos in the middle.]
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,
4. Attached to a carbon atom located between two hereoatoms
in a heterocyclic ring.