E-Z notation

E-Z notation, or the E-Z convention, is the IUPAC preferred method of describing the stereochemistry of double bonds in organic chemistry. It is an extension of cis/trans notation that can be used to describe double bonds having three or four substituents.
Following a set of defined rules (Cahn-Ingold-Prelog priority rules), each substituent on a double bond is assigned a priority.
If the two groups of higher priority are on opposite sides of the double bond, the bond is assigned the configuration E (from entgegen, German: [ɛntˈɡeːɡən], the German word for "opposite").
If the two groups of higher priority are on the same side of the double bond, the bond is assigned the configuration Z (from zusammen, German: [tsuˈzamən], the German word for "together").

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(E)-But-2-ene

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(Z)-But-2-ene