Two or more substances that are composed of the same elements in the same proportions but differ in properties because of differences in the arrangement of atoms. Cis- and trans-isomerism, for example, refers to the arrangement of atom groups adjacent to a carbon-carbon double bond (C=C); in the ‘cis-isomer’ the groups are on the same side of the double bond, while in the ‘trans-isomer’ they are on opposite sides.