A diastereomer is a type of stereoisomer that is not a mirror image of another molecule. Stereoisomers have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of those atoms. Diastereomers typically have different physical and chemical properties, making them distinct from each other. Unlike enantiomers, which are mirror images and have identical physical properties, diastereomers can exhibit variations in boiling points, melting points, and reactivity. This difference arises because diastereomers have multiple chiral centers, such as in amino acids or sugars, leading to a variety of spatial arrangements.