Shūnyam is the formal Sanskrit for Śūnya in common-speak. Etymologically, Shūnyam originate in the notion of hollowness, of ‘Empty Inside’.
The term ‘Empty’ or ‘Null’ as used in English translations of Shūnyam originate directly from the vocabulary of Classical Logic as does the word ‘Form’ as used in the Heart Sutra.
[Although no guru, fee-speaker or book-writer I have met is aware of this root, which might explain their wildly creative interpretations of these two terms.]
Shūnyam itself is not to be confounded with the numerous versions of Śūnya with a suffix that evolved in the regional Dharmic literatures well into the 10th Century [ŚūnyaBrahman, ŚūnyaPurusha et al].
Or the selective use of the term Shūnyathā in others, from the early Theravada Schools to the later Mahayana appropriations.
The 25 Sloka compacting of the MahaPrajñāpāramithā is the Hridaya Sūtra [‘Heart Sūtra’]. It is to be read and interpreted only as supplement to the Vajrachedika for its very tight phrasing can seriously miscue the entrant. If the Diamond Sutra is loopy, circuitous, the Heart Sutra, is in-your-face direct.
I have chosen the Nalanda Committee Translation Translation although numerous other European language translations from Max Muller to Edward Conze going back over a century are available on the Web.
The pivotal line from the Heart Sūtra which the Nalanda Translation Committee wisely decided to leave in the original Sanskrit [and I, less wisely, have chosen to translate] is the following:
‘The Prajñā-Pāramithā Mantra is said in this way: Om Gatheh! Gatheh! ParaGatheh! ParasamGatheh! Bodhi Svaha!’
‘My understanding, released, soars, takes flight. Svaha!‘. In contemporary language: The [Self-Scuttling] Sight-Insight on the very nature of Sight-Insight. In other words, Shūnyam.
The Sūtra approaches this axial verse by introducing a celebrated line: ‘Form is not different from Emptiness; Emptiness, from Form‘. And to do this it introduces two words: Shūnyathā and [Nama] Rupam translated into English as Emptiness and Form.
Few texts comes closer to the truth of Shūnyam than the Heart Sūtra. Nāmarūpa, an assertive noun is about the closest conventional term you will find to the free, open-ended negation: ‘Not-True Nothing’.
The later derivative construct of Shūnyathā found its inspiration in an established earlier divide: Táthātā and Tát. What’s the difference?
Táthātā, typically translated into English as ‘That-ness’ [also as Suchness, Thusness] is the abstraction of Tát [‘That’].
If it had any other purpose it was to differentiate the philosophical substance of ‘That’ from its routine grammatical chores. This would have been entirely legitimate.
But it was rarely used in this service and very soon after its construction [and predictably so], ‘That-ness’ took on a philosophical life of its own. You can see this as much in the early Vedanthic literature as in the later Buddhist texts.
What happened with Tát: Táthātā is exactly what happened much later in the construction of the distinction: Shūnyam: Shūnyathā. Shūnyam is to ‘That’ as Shūnyathā is to ‘That-ness’.
[But the derivative extension of Shūnyathā continued to firmly retain the original Logical Form of its parent even if never effected. It’s most consistent definition in the higher texts has been ShūnyathāShūnyathā: ‘The Emptiness of Emptiness itself’, a full-blown Self-Eating Expression.]
The result was consequential. Depending on the source, Shūnyathā has been interpreted both as a synonym for a conceptualized Shūnyam and otherwise as a mystic expression for some special capture of Awareness, Consciousness, or a muddled ‘Whole’.
I sat in once on a lecture by a celebrated Tibetan monk who went on about ‘Stages of Emptiness’. To hold forth, as I have seen, about the emptiness of tables and trees in a literal take on their supposed hollowness is to entirely miss the point. The table and tree may be hollow or full. It is the observer not the observed that is ‘Empty’.
[The literature, especially the contemporary takes, pivot off the Principle Of Co-Dependence without reaching into the Axioms of Sight and ‘Self” in exact parallel to the Vedanthic stop at ‘Being: Consciousness’.]