The terms saturation and unsaturation refer to the type of bonding between the carbon atoms in a structure. A molecule that contains only carbon-carbon single bonds is said to be saturated. Double bonds and triple bonds are said to be unsaturated.
oct-3-ene – C8H16
When a molecule contains many double (or triple) bonds it is said to be polyunsaturated. In long chain hydrocarbons unsaturation gives rise to ‘kinks’ in the structure and prevents the long chains aligning easily. This lowers the melting point.
The degree of unsaturation can be determined using reagents that react quantitatively with the double bonds.