The name sept is given to members of a clan who do not share its name, although they may or may not be of the same blood.
Within a clan, following the Highland fashion of designating people by the names of their fathers, grandfathers, and sometimes more remote ancestors, other names could be used for certain family groups. Hence, in Clan Campbell we have the MacTavish (= "Son of Thomas" in Gaelic) sept, descended from a Thomas Campbell. The MacConochie (= "Son of Duncan", in Gaelic) sept, descended from a Duncan Campbell, and early offshoots like the MacArthurs and the MacIvers who descend from the chiefly stock before the adoption of the name Campbell.
The word Clann in Gaelic means no more than "family" and there were hundreds of such groups who made no pretence to set up as major powers on their own but who followed the local chief and became members of his clan.
Sometimes these smaller kindred were widely spread and their branches could follow different Chiefs. And very often the same name could come from a whole range of unrelated sources particularly in the case of Mac-names or patronymics, as they are called, which mean "Son of".
The 19th century enthusiasm for clans, fostered for their own reasons both by the tartan manufacturers and the Clan Societies, resulted in the attribution of as many names as possible to clans as septs -- sometimes with ludicrous results. The idea that all Millers should belong to Clan Macfarlane or all Taylors to Clan Cameron is clearly untenable; this is not to say that the names were not used by members of those clans on occasion, but they are both work-names of trades practiced in practically every community across English-speaking Britain. Nor is the suggestion that all sons of Harry, Gib, Thomas or Arthur should descend from the same person of that particular name any more tenable.
Our list of septs is by no means perfect; there are some names whose inclusion would seem to be due more to this sept-hunting enthusiasm than to historical accuracy and there are many names which loyally followed the Campbell Chiefs for centuries which have not been included.
But rather than encourage still further confusion, our Chief has said that he does not wish to make any alterations to the "official" list of Campbell sept names which follows.
Rather than do that, he said some years ago that he was prepared to accept as members of Clan Campbell all those of Scottish descent who were prepared to acknowledge him as their Chief. This very much follows what actually happened in past times.