Illustrated Glossary of Organic Chemistry

Homolog: Structurally-similar molecules within a series with the same functional group but different numbers of some repeated groups such as CH2. This series of molecules is called homologous series.

CH3OH

CH3CH2OH

CH3CH2CH2OH

CH3CH2CH2CH2OH
Methanol
  
Ethanol
  
1-Propanol
  
1-Butanol
A homologous series of alcohols which differ in the number of methylene groups.