A principal expression of Vedic insight is Tát [literally, ‘That’]. The Summum Bonum of popular, if redacted Dharmic Teaching is: Tát Tvam Asi, the same Tát [‘That’] of the Rig Veda. In translation: ‘You are That’.
‘That’ is an Expression of Inexpressibility. A self-scuttling assertion in negation, an immediate, unregenerate self-contradiction. It is neither noun nor verb, is grammatically homeless, a lexicographer’s nightmare, and meant to be so.
No identities, aspects, elements, endowments, features, qualities, temperament, tendencies. You may not source it for ethical or social directives [rules], go philosophical or poetic on its attributes…
Point a finger, draw a line, a thought, emote a feeling towards ‘That’ and by that very act, what you have pointed a finger to, drawn a line of, thought, emoted, is not ‘That’.
‘You may not express ‘That’. Anything you express as ‘That’, by that very fact, is not ‘That”. To even call it ‘Inexpressible’ violates its inexpressibility.
‘That’ has the exact same Logical Form as the Symbol ‘0’. It preceded its birth by as much as a millennium.
‘That’, is red-meat for the book-read and the religious-minded.
I have seen it translated as a word for ‘The Supreme’, ‘The Infinite’, the ‘One True God’, the ‘True Inner Self’ and numerous other variations all of which are meant to reign in its implacable inexpressibility to the domain of the comforting and the familiar.
If it was ‘The Supreme’ it would simply be called ‘The Supreme’. The vocabulary is rich enough. Of course, in every case these breezy claims are only made by one standing a mile away from The-Not.