Double bond isomers are always considered diastereomers, not enantiomers. Diastereomerism can also occur at a double bond, where the cis vs trans relative positions of substituents give two non-superposable isomers. Many conformational isomers are diastereomers as well.
In the case of diastereomerism occurring at a double bond, E-Z, or entgegen and zusammen (German), is used in notating nomenclature of alkenes.