Definition of Diastereomers
Diastereomers are the molecules which are stereoisomers of each other. They are not the mirror images of one another as in the case of enantiomers and they are non-superimposable on one another. If one of the two molecules is flipped 180 degrees, the stereochemistry of both the molecules becomes different.
The stereoisomers which have two or more stereocenters present are diastereomers. If the diastereomers are different from each other at only one stereocenter, then the molecules are known to be epimers. Moreover, the diastereomers have different physical properties. Their reactivities, melting and boiling points and densities are different from one another.