Diene

In organic chemistry, a diene ( ) or diolefin ( ) is a hydrocarbon that contains two carbon pi bonds. 
Conjugated dienes are functional groups, with a general formula of CnH2n-2. Dienes and alkynes are functional isomers. Dienes occur occasionally in nature but are widely used in the polymer industry.

Classification
Dienes can be divided into three classes, depending on the relative location of the double bonds: 
Cumulated dienes have the double bonds sharing a common atom as in a group of compounds called allenes.
Conjugated dienes have conjugated double bonds separated by one single bond.
Unconjugated dienes have the double bonds separated by two or more single bonds. They are usually less stable than isomeric conjugated dienes. 

Compounds that contain more than two double bonds are called polyenes. Polyenes and dienes, share many of their properties.


Cycloaddition
An important reaction for conjugated dienes is the Diels–Alder reaction.  Many specialized dienes have been developed to exploit this reactivity for the synthesis of natural products, e.g. Danishefsky’s diene.

Other websites
IUPACs Gold Book definition of dienes dienes PDF