A saturated compound contains only single bonds and therefore holds the maximum possible number of hydrogen atoms, while an unsaturated compound contains one or more multiple bonds and so carries fewer hydrogens. The distinction matters because multiple bonds are reactive sites, so unsaturated compounds undergo addition reactions that saturated ones do not. For example, an alkane is saturated, whereas an alkene or alkyne is unsaturated.

饱和化合物只含单键,因而具有尽可能多的氢原子;不饱和化合物含有一个或多个多重键,因而所含的氢较少。这一区别之所以重要,是因为多重键是反应活性部位,故不饱和化合物能发生饱和化合物所不能发生的加成反应。例如,烷烃是饱和的,而烯烃或炔烃是不饱和的。