A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called homologous series.

Consider the homologous series of alcohols – methanol, ethanol, etc. Note that methanol (CH₃OH) is obtained by replacing one of the H atoms with –OH group. Similarly, the –OH group replaces one of the H atoms of ethane (CH₃CH₃) to form ethanol
(CH₃CH₂OH) and so on. The chemical properties of these compounds are similar. The successive members of the homologous series differ by a –CH₂ unit.

For example, CH₃OH and CH₃CH₂OH, CH₃CH₂OH and CH₃CH₂CH₂OH differ by a –CH₂ unit.

Also, members of a homologous series can be represented using a general formula. In the case of alcohols, the general formula is C_nH_2n+1O, where ‘n’ is the number of carbon atoms.