A diene is a compound that contains two carbon–carbon double bonds (two C=C bonds). Dienes matter because when the two double bonds are conjugated, that is, separated by a single bond, their π systems overlap and the molecule shows special stability and reactivity, as in the formation of rubber and many synthetic reactions. A conjugated diene contrasts with an isolated diene, in which the two double bonds are too far apart to interact.

二烯烃是含有两个碳碳双键(两个 C=C 键)的化合物。二烯烃之所以重要,是因为当两个双键共轭(即彼此被一个单键隔开)时,它们的 π 体系相互重叠,使分子表现出特殊的稳定性和反应活性,如在橡胶的形成及许多合成反应中。共轭二烯与孤立二烯形成对照,后者的两个双键相距太远而无法相互作用。