Are #cis# and #trans# isomers examples of diastereomers?
Yes, cis and trans isomers are examples of diastereomers.
We usually think of diastereomers as optical isomers that are not mirror images of each other.
But a diastereomer is any stereoisomer that is not an enantiomer.
The cis and trans isomers of but-2-ene are isomers that are not mirror images of each other, so they are diastereomers.