Isomers that are related to each other by either rotations of double bonds, or alterations of chiral centres are stereoisomers.

Stereoisomers are molecules which differ only in the three-dimensional arrangement of their constituent atoms

Molecules with the same atoms and bond structure, but different three dimensional arrangements of atoms. For example, the CH3 groups in CH3CH=CHCH3 can be placed on the same side of the double bond in one stereoisomer and on opposite sides in another.