What is "structural isomer"

Isomer \I"so*mer\ ([imac]"s[-o]*m[~e]r), n. [See Isomeric.] (Chem.) A compound which is isomeric with another body or compound; a compound having the same chemical composition as another compound; a member of an isomeric series.

Note: An isomer may be a structural isomer, in which some of the atoms are bonded to different atoms (as in CH3.CH2.NH2 and CH3.NH.CH3), or an optical isomer, in which the number and types of chemical bonds are identical, but the arrangement of the atoms in three-dimensional space is different; the latter are also called stereoisomers.

n. (context chemistry English) either one of a number of compounds that exhibit structural isomerism

Structural isomerism, or constitutional isomerism (per IUPAC), is a form of isomerism in which molecules with the same molecular formula have bonded together in different orders, as opposed to stereoisomerism. There are multiple synonyms for constitutional isomers.

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