It refers to the bai two reactants of the Diels-Alder reaction. Among them, the olefin zhi that provides the conjugated du double bond is called the diene, and the dao olefin that attacks the diene is the dienophile: such as ethylene and 1,3 -Butadiene undergoes Diels-Alder reaction to generate cyclohexene, where ethylene is a dienophile, and 1,3-butadiene is a dienoid.