Structural isomerism, or constitutional isomerism (per IUPAC), is a form of isomerism in which molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s with the same molecular formula have bonded together in different orders, as opposed to stereoisomerism

Stereoisomerism

Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms , but that differ only in the three-dimensional orientations of their atoms in space. This contrasts with structural isomers, which share the same molecular formula, but the bond connections...

. There are multiple synonyms for constitutional isomers.

Three categories of constitutional isomers are skeletal, positional, and functional isomers. Positional isomers are also called regioisomers.