A convention for the description of a molecule showing cis-trans isomerism (see isomerism). In a molecule ABC=CDE, where A,B,D, and E are substituent groups, the sequence rule (see CIP system) is applied to the pair A and B to find which has priority and similarly to the pair C and D. If the two groups of highest priority are on the same side of the bond then the isomer is designated Z (from German zusammen, together). If they are on opposite sides the isomer is designated E (German entgegen, opposite). The letters are used in the names of compounds; for example (E)-butenedioic acid (fumaric acid) and (Z)-butenedioic acid (maleic acid). In compounds containing two (or more) double bonds numbers are used to designate the bonds (e.g. (2E, 4Z)-2,4-hexadienoic acid). The system is less ambiguous than the cis/trans system of describing isomers.
http://www.acdlabs.com/iupac/nomenclature/93/r93_626.htm Information about the convention from IUPAC