The Collaborative International Dictionary
Meso- \Mes"o-\, Mes- \Mes-\ [Gr. me`sos in the middle.]
A combining form denoting in the middle, intermediate.
specif. (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; as, meso-tartaric acid.
Attached to a carbon atom located between two hereoatoms in a heterocyclic ring.
Douglas Harper's Etymology Dictionary
word-forming element meaning "middle, intermediate, halfway," sometimes mes- before vowels, comb. form of Greek mesos "middle, in the middle; middling, moderate; between" (see medial (adj.)).
Wiktionary
pre. 1 middle 2 intermediate 3 (context chemistry English) Designating an achiral member of a group of diastereoisomers that has at least one chiral member