‘Not-Two’: Àdvaitham, a term that long predates Shūnyam. Yājñavalkya defined it as simply: ‘ Neither before nor after; Neither inside, nor outside’.
It has lots of layers to it and you won’t really notice them until you slip on one. But the three most relevant can be readily listed.
The first is the assumption of the ‘Independent and Separate ‘Self’. The ‘Two’ of the ‘Subject-Object Divide’. [‘Self and World’; ‘God and Man’, and so on.]
Secondly, the open-ended: ‘Not’.
Thirdly, our reflexive tendency to abstract in Sign and Symbol [‘Doubles that Refer’] and hence make our World amenable to Logic and Language.
In particular, expressions formulated as ‘Sign’, and further extended in Logic, Language and ‘Thought’. And then cheerfully contracted or expanded until we get seriously lost.
The earliest expansion of ‘Not-Two’ in the literature is as: ‘One without a Second’. ‘The One’ [Sanskrit: Ekam Sat] can be found in the history of every literate tradition.
From ‘The One’ of Plotinus that was the mainstay of the hugely influential European Neo-Platonic tradition with roots in the Parmenides to that of that of the Abrahamic Faiths [which gets conceptualized and reified into a later ‘Monotheism’.]
Shūnyam is not about the usual binaries of True and False or Right and Wrong. Absence and Presence. Emptiness and Fullness. Null and Whole, ‘Being’ and Consciousness. These are inexact but intuitively helpful beginning conceptual pointers that exit the scene once their work is done.
The word ‘Two’ has somehow managed to hold on to its clothes, keep its identity over the many centuries and continents it has crossed. I know of no other word quite like it [ well, perhaps ‘Ma’ and ‘Pa’]. Dvi [Sanskrit]; Duo [Latin]; Dio [Greek]; Do [Persian]; Tvau [Norse]; Tvee [Dutch]; and you can guess ‘Zvei’ and, ‘Deux‘.