A homologous series is a series of compunds in which one compound differs from a preceding one by a  - CH_{2} - group. Members of the homologous series are called homologs.

Characterisitcs of homologous series:

1. Each homologous series is represented by a general formula. For example alkane series has a general formula   C_{n}H_{2n+2}
2. Members of a homologous series have similar chemical properties
3. The physical properites change through out the series in a regular way. For example, melting points and boiling points increase in the series from methane to decane
4. Members of the same homologous series can be prepared by the general methods of preparation.

https://en.wikipedia.org/wiki/Homologous_series