Isomer

An isomer (Greek isos the same and meros a part) of a compound is any other compound having an identical simple formula (that is to say, ratio of constituent elements) but a different structure. Different isomers have different properties and thus cannot be considered to be the same substance. The simplest form of isomerism, called structural isomerism, or constitutional isomerism, occurs when atoms combine in different arrangements.

Example
A simple example of isomerism is given by propanol: it has the formula C3H8O (or C3H7OH) and occurs as two isomers: propan-1-ol (n-propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II)