In chemistry, a structural isomer (or constitutional isomer  in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term metamer was formerly used for the same concept.
For example, butanol , methyl propyl ether , and diethyl ether  have the same molecular formula  but are three distinct structural isomers.
The concept applies also to polyatomic ions with the same total charge.  A classical example is the cyanate ion  and the fulminate ion . It is also extended to ionic compounds, so that (for example) ammonium cyanate   and urea  are considered structural isomers, and so are methylammonium formate  and ammonium acetate .
Structural isomerism is the most radical type of isomerism.  It is opposed to stereoisomerism, in which the atoms and bonding scheme are the same, but only the relative spatial arrangement of the atoms is different. Examples of the latter are the enantiomers, whose molecules are mirror images of each other, and the cis and trans versions of 2-butene.
Among the structural isomers, one can distinguish several classes including skeletal isomers, positional  isomers (or regioisomers), functional isomers, tautomers, and structural topoisomers.