Isomerism refers to the phenomenon where molecules with the same molecular formula have different arrangements of atoms in space. There are two main types of isomerism: constitutional isomers, which differ in the connectivity of their atoms, and stereoisomers, which vary based on their spatial orientation. Stereoisomers can further be divided into enantiomers, which are non-superimposable mirror images, and diastereomers, which are not mirror images and include cis and trans configurations.