At the time when these chapters [the last chapters of The Synthesis of Yoga] were written, the name “overmind” had not been found, so there is no mention of it. What is described in these chapters is the action of the supermind when it descends into the overmind plane and takes up the overmind workings1 and transforms them. It was intended in later chapters to show how difficult even this was and how many levels there were between human mind and supermind and how even supermind, descending, could get mixed with the lower action and turned into something that was less than the true Truth. But these later chapters were not written.
The distinction [between the Supermind and the Overmind] has not been made in the Arya because at that time what I now call the Overmind was supposed to be an inferior plane of the Supermind. But that was because I was seeing them from the Mind. The true defect of Overmind, the limitation in it which gave rise to a world of Ignorance, is seen fully only when one looks at it from the physical consciousness, from the result (Ignorance in Matter) to the cause (Overmind division of the Truth). In its own plane Overmind seems to be only a divided, many-sided play of the Truth, so can easily be taken by the Mind as a supramental province. Mind also when flooded by the Overmind lights feels itself living in a surprising revelation of divine Truth. The difficulty comes when we deal with the vital and still more with the physical. Then it becomes imperative to face the difficulty and to make a sharp distinction between Overmind and Supermind — for it then becomes evident that the Overmind Power (in spite of its lights and splendours) is not sufficient to overcome the Ignorance because it is itself under the law of Division out of which came the Ignorance. One has to pass beyond and supramentalise Overmind so that mind and all the rest may undergo the final change.
— Sri Aurobindo, Letters on Yoga — I, CWSA Vol. 28, pp. 148, 49
1. The highest Supermind or Divine Gnosis existent in itself is something that lies beyond still and quite above.