From Rangjung Yeshe Wiki - Dharma Dictionnary
Akanishtha ('og min). The 'highest;' the realm of Vajradhara, the enlightened sphere the dharmakaya buddha. Often used as a synonym for 'dharmadhatu.'
For a discussion of the various types of Akanishtha, see Gyurme Dorje's translation of Longchen Rabjam's phyogs bcu mun sel.