Structural isomer

Structural isomerism, or constitutional isomerism, is a form of isomerism in which molecules with the same molecular formula have bonded together in different orders, i.e. they each have a different structural formula, as opposed to stereoisomerism.

Quotes

 * Constitutional isomerism is not limited to alkanes—it occurs widely throughout organic chemistry. Constitutional isomers may have different carbon skeletons (as in isobutane and butane), different functional groups (as in ethanol and dimethyl ether), or different locations of a functional group along the chain (as in isopropylamine and propylamine). Regardless of the reason for the isomerism, constitutional isomers are always different compounds with different properties but with the same formula.
 * John McMurry, Organic Chemistry 8th ed. (2012), Ch. 3 : Organic Compounds: Alkanes and heir Stereochemistry