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[Ger. entgegen opposite] a stereodescriptor used to specify the absolute configuration of rigid compounds, such as those having double bonds. The substituents attached to the double-bonded carbons are ranked according to the Cahn-Ingold-Prelog sequence rules; then if the higher priority substituents are on the same side of the double bond the configuration is Z, otherwise E. In the simple case when both carbons have the same pair of substitutents, Z- is equivalent to cis-, E- to trans-.Medical dictionary. 2011.