What is the meaning of the term 'structural isomers'?

A pair of strutural isomers are two molecules which have the same molecular formula; the same number of each atom, but which have these atoms arranged differently, and are therefore distinct molecules. An example of this would be 2-propanol and 1-propanol. Both have the formula C3H8O, but the alcohol -OH group is attached to an end carbon (position 1) in 1-propanol and to the middle carbon (position 2) in 2-propanol. Despite the molecular formula remaining the same, these two strutural isomers will have different physical as well as chemical properties, and are considered completely distinct species.