meso-

from The Century Dictionary.

• A combining form used in organic chemistry to indicate optical inactivity through internal compensation. See mesoform.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• prefix middle
• prefix intermediate
• prefix chemistry Designating an achiral member of a group of diastereoisomers that has at least one chiral member