A meso compound or meso isomer is a chemical compound with molecules that contain 2 or more asymmetric atoms (stereocenters) but which is optically inactive (or achiral) because it contains an internal plane of symmetry.
For example one of the isomers of tartaric acid which is depicted on the right is a meso compound. An internal mirror plane exists bisecting the molecule; on rotating the molecule 180° on a plane perpendicular to the screen (that is, superimposing the bottom OH with the top one), the same apparent stereochemistry is obtained. (see Fischer projection.)
For a compound with multiple asymmetric carbons, an internal plane of symmetry indicates that the compound is achiral. However, it is still possible for a compound with multiple asymmetric carbons and no internal plane of symmetry to be meso. If there is only one asymmetric carbon, then the compound must be chiral.