A series of compounds having the same functional group but a difference of $-C{H}_2$ unit (14 unit mass) between two successive members. They show same chemical properties and gradual variation in physical properties. The group of compounds is known as:

A series of compounds having the same functional group but a difference of $-C{H}_2$ unit (14 unit mass) between two successive members are known as homologous series. They show the same chemical properties and gradual variation in physical properties.

Ex: All alkanes fall under homologous series.