MacMillan, Macmillan, McMillan, and M'Millan are variants of a Scottish surname; see also the similar surname McMillen.
The progenitor of the clan was said to be Airbertach, Hebridean prince of the old royal house of Moray.
Airbertach had a son named Cormac, who was a bishop, and Cormac's own son Gilchrist, or in Gaelic, Gille Chriosd, the progenitor of the Clann an Mhaoil, was a religious man like his father.
Because of this, Gille Chriosd wore the tonsure, which gave him the nickname Maolan or Gillemaol.
The name MacMillan thus literally means, "son of the tonsure".