The existence of two or more compounds with same molecular formula but different properties (physical, chemical or both) is known as isomerism; and the compounds themselves are called isomers.
The term was given by Berzelius.
The difference in properties of two isomers is due to the difference in the arrangement of atoms within their molecules. Isomerism may be of two types:
A. When the isomers differ only in the arrangement of atoms or groups within the molecule, without any reference to space, these are known as structural isomers
and the phenomenon as
e.g. The formula C4H10 represents two possible structural formulae, butane and methylpropane
B. Stereoisomers have the same structure and bond order but their atoms and groups of atoms are arranged differently in space. They have different spatial arrangements and their molecules are not superimposable. There are two types