Homologous series: A series of organic compounds in which all members have the same general formula, similar chemical properties, and successive members differ by a $—CH_2$ group.

Characteristics of Homologous Series:
1. Same General Formula: All members of a series can be represented by a common general formula (e.g., alkanes $C_nH_{2n+2}$, alkenes $C_nH_{2n}$).
2. Successive members differ by $—CH_2$ group: The molecular formula of any two consecutive members differs by a $—CH_2$ group and 14 amu (atomic mass units).
3. Similar Chemical Properties: Due to the presence of the same functional group, all members of a homologous series show similar chemical properties.
4. Gradation in Physical Properties: There is a gradual change in physical properties (e.g., melting point, boiling point, density) as the molecular mass increases due to increasing van der Waals forces.
5. Preparation Methods: All members of a homologous series can be prepared by similar general methods.