不饱和烃是含有碳-碳双键或三键的烃。

如果烃分子中只有单键，则叫做饱和烃，像下式，属于饱和烃里面的饱和链烃，饱和链烃称为烷烃，烷就是“完”全的意思，分子内只有单键且成链状。

如果各键没有被氢原子占满，富余的键会和其他碳原子组成双键或三键，就是不饱和烃，如下式：

含有一个或多个双键的叫“烯”，是氢原子“稀”少的意思；含有一个或多个三键的为“炔”，是氢原子“缺”乏的意思。

Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms. Those with at least one carbon to carbon double bond are called alkenes and those with at least one carbon to carbon triple bond are called alkynes. The position of the double or triple bond is shown by a number written either at the start of the name, or just before the -ene or -yne suffix (e.g. pent-2-ene and 2-butyne). The number represents the position of the first of the two carbons making the bond, in the longest carbon chain. Alkenes and alkynes with more than one double or triple bond respectively are named with a prefix preceding the -ene or -yne (e.g. 2,4-pentadiene).