How to Identify Stereoisomers of an Alkene
Alkenes, which are molecules containing carbon-carbon double bonds, have the possibility of having stereoisomers, just as ring systems do. This is because, unlike carbon-carbon single bonds, which are free to rotate, double bonds are fixed and rigid. In other words, rotation around carbon-carbon double bonds is not possible at reasonable temperatures.
Molecules that have the same connectivity of atoms, but different orientations of those atoms in space are called stereoisomers.