Complex sentence: A meso compound or meso isomer is a stereoisomer with an identical or superimposable mirror image i.e a non-optically active member of a set of stereoisomers, at least two of which are optically active.
Split into simpler sentences: A meso compound or meso isomer is a stereoisomer with an identical or superimposable mirror image. A non-optically active member of a set of stereoisomers, at least two of which are optically active.