What type of isomers are cis-but-2-ene and 2-methylpropene?
(2​Z)-But-2-ene and 2-methylprop-1-ene are constitutional isomers as they have different connectivities, i.e. the sequence in which the atoms are bonded to each other is not the same. For example, the carbons highlighted here are different: in (2​Z)-but-2-ene it is only bonded to two other carbons (through a single bond and a double bond), but in 2-methylprop-1-ene it is bonded to three other carbons:



Cis/trans isomers are a subset of stereoisomers, which need to have the same constitution, or molecular connectivity. The two isomers of but-2-ene satisfy this:



Note how both highlighted carbons are now bonded to the same partners, but with a different spatial arrangement.