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I know that we have two CH3 groups on one side and restricted rotation around the double bond but aren't we supposed to have two similar groups on the other side as well ? Does this mean that we only need one side of the molecule to have identical groups/atoms in order to call it a cis isomer ?
With the same reasoning as provided here, I recommend that you name this compound (2 E)-2-bromobut-2-ene.
Using (cis/trans) may be useful around an isolated double bond with only two substitutents. While in the example presented by you the two methyl groups are on the same side of the double bond, they are not the only ones to be considered here. Triple and quadruple substituted dienes should be named in accordance to the CIP rules only.
Edit by 02-Jan-2019: Agreeing with a comment by @Loong, the previously suggested name, "(E)-bromo-2-butene", was replaced by "(2 E)-2-bromobut-2-ene". This edit removes simultaneously the ambiguity about the position of the bromine, as well as the double bond. The interested is referred to the illustrated examples of rule R-7.1.2 of the corresponding Blue Book here.
