A homologous series is a group of organic compounds having similar structures and similar chemical properties in which the successive compounds differ by ${\mathrm{CH}}_2$ group.
Example : Alkanes with general formula ${\mathrm{C}}_{\mathit{n}}{\mathrm{H}}_{2\mathit{n}+2}$
Characteristics :
(i) All the members of a homologous series can be represented by the same general formula and they have same functional group.
(ii) Any two adjacent homologues differ by 1 carbon atom and 2 hydrogen atoms in their molecular fomulae.
(iii) The difference in the molecular masses of any two adjacent homologues is $14\mathrm{u}$.