eg. 1: Butane and isobutane have the same molecular formula, C4H10, but different structural formulas.
Therefore, butane and isobutane are constitutional isomers.
eg. 2: Ethyl alcohol and dimethyl ether have the same molecular formula, C2H6O, but different structural formulas.
Therefore, ethyl alcohol and dimethyl ether are constitutional isomers.
see also stereoisomers, skeletal isomers, positional isomers, functional isomers