In Panosophy, in conformity with the Hidden Harmony, Tathatā is the dual of Śūnyatā, the central notions of ‘Suchness’ and ‘Void’ in Mahāyāna and Hīnayāna Buddhism, as ‘Great Vehicle’ and ‘Small Vehicle’, respectively.
These Buddhist distinctions indicate that we cannot avoid the fundamental law of the Universe even when seeking words that denote the Ineffable Nonduality of the Absolute.
In the case of Tathatā, this is viewed as the Plenum or Fullness, as the Totality of Existence experienced as indivisible Wholeness, beyond all concepts and distinctions.
Sanskrit ‘suchness, thusness, thatness’, from tathā ‘so, thus, in this way’, from tat ‘that’, and -tā ‘-ness’, from PIE base *tod-, extended form of *to-.