Elements of Unsaturation
Alkenes are unsaturated because they are capable of adding hydrogen in the presence of a catalyst. Alkanes are saturated because they cannot react with anymore hydrogen.
Each pi bond of an alkene (or alkyne) or the ring of a cyclic compound decreases the number of hydrogen atoms in a molecular formula. This feature is called “element or degree or un-saturation”.
Each element of unsaturation corresponds to two fewer hydrogen atoms than the “saturated” formula.