ôSept " is an Irish term used to explain the use of a variety of surnames by members of a single clan. In Ireland, "sept" is roughly synonymous with the Scottish "clan" and refers to an inter-related family.
The variety of surnames within a Scottish clan does not represent separate and definable sub-clans but instead the transitions of the Gaels into the English naming system or through marriage, migration or occupation. The main family itself may have developed a variety of surnames.
Avoid the use of the term "sept", describe these names as what they are surnames of the family and of allied and dependent families. It is preferable to speak of "The names and families of Clan MacPhail" rather than to call a name "a sept of Clan Macphail".