A homologous series is a series of compounds with the same general formula, usually varying by a single parameter such as the length of a carbon chain. Compounds within a homologous series typically have a fixed set of functional groups that gives them similar chemical and physical properties.
For example, methane, ethane, propane, butane, etc. are all part of the alkane homologous series. The general formula of this series is CnH2n+2.
Methane , CH4
Propane , CH3CH2CH3
Butane , CH3CH2CH2CH3
It can be noticed that there is a difference of −CH2 unit between each successive compound.