Brahman

 “One only, without a second” (Chandogya Upanishad 6.2.1). Thus all the Shrutis proclaim. Infinite, Absolute, Eternal, Changeless, the All, is THAT, without attributes, without qualities, beyond name and form: Nirguna Brahman.

What the Upanishads say

“Then was not non-existence nor existence… THAT only breathed by its own nature: apart from THAT was naught” (Rig Veda 10.129.1-2). It contains all, therefore can no particular thing be said of It. It is all, therefore can no one thing alone be ascribed to It. It is not Being only, for that would exclude Non-Being; but Being arises in It, and Non-Being is also there.

“When no darkness (was), then (there was) not day nor night, nor being nor not-being, (but) the Blessed alone” (Shwetashwatara Upanishad 4:18). The same upanishad (5:1) says: “In the imperishable infinite supreme Brahman knowledge and ignorance are hidden.” “It is,” (Katha Upanishad 2.3.12