Daath

Daath or Da'at in the Kabbalah, is the location where all ten spheres in the Tree of Life are united as one.

It means Hidden Word or Knowledge. In Daath all spheres or sephirot exist in their perfected state of infinite sharing. The three sephirot of the left column that would receive and conceal the Divine light, instead share and reveal it. Since all sephirot radiate infinite self-giving Divine Light, it is no longer possible to distinguish one sephira from another, thus they are one.

Daath is not always depicted in representations of the sefirot, and could in a sense be considered an "empty slot" into which the gem of any other sefirot can be placed. The Divine Light is always shining, but not all humans can see it. Hence the meaning "Hidden Word' The concealment or revelation of the Divine Light shining through Daath does not actually happen in Daath itself. It only seems that way from the human perspective within Malchuth. The perception of change can only occur in Malchuth. Humans who become in service to the light become able to see it, and for them the benefits of Daath's light seem "revealed". However, humans who remain in service to self cannot see it, and for them its benefits remain "hidden".

Reference
Wikipedia Da'at