Svabhāva, traditional Chinese Buddhist text translated as "自性", i.e., Self-nature. A Svabhava is a Thing-of-itself, it exists on it's own, cannot be deconstructed by the Eight Conditions quested by Nagarjuna; it is not caused by Causal-correlations (因緣), nor Compounded (和合). However, the world is made by (1)Causal-correlations and (2)Compounding, of every phenomena that the intellectual mind able to conceive. Therefore, it is empty. Because anything that's coming from Causal-correlations or Compounding, when investigating by the intellectual examination incisively with a superior mind, the result is - unobtainable. For nothing is obtainable therefore, the term to address it: "Emptiness".
My elaboration is, Causal-correlations and Compounding shorted as Correlative-arising (緣起). In my investigation, "dependent-arising", or most of the English translated terms I came across are, questionable; creating unnecessary hindrances to understand what precisely it meant.
There are four types of Causal-correlations, Sanskrit: Catvārah-pratyayāh (四緣). Nagarjuna listed and deconstructed each as:
(the translated terms are my designations)
a) Correlative-cause (因緣): Cause and effect. Deconstructed as: an effect not appeared the cause can't be defined; if the effect not in the cause, the cause cannot cause the effect, but if the effect already in the cause the effect doesn't need the cause for it existed already... etc.
b) Consecutive-cause (次第緣): A temporal cause, i.e., with the factor of time. Deconstructed as: if the effect not appeared, the cause can't disappear; however if the cause not disappeared how could the effect appear... etc.?
c) Causative-cause (緣緣): A spatial cause, i.e., with the factor of another object. Deconstructed as: for due to a) the cause and effect cannot be established therefore not a "thing" there, there's no object to give as a cause.
d) Superlative-cause(增上緣): All the above causes multiplied. Deconstructed as: since the above three can't be established this also null.
I'm afraid Jay Garfield's understanding of the Catvārah-pratyayāh which a very basic Abhidharma concept so questionable in his The Fundamental Wisdom of The Middle Way; I read his chapter 10, then 1 and 2, put aside.
(2) Compounding Compounding is unobtainable. For if things are different, they can't be compounded to be one thing; yet if they are the same, can't be compounded either, for same thing doesn't compound :). (I was laughing all the way when reading Madhyamaka (Classical Chinese translated by Kumarajiva) the 1st time, now I can't help laughing again when writing at here :))... difference due to differing therefore differences, different object unobtainable... etc.
Since all phenomena of the world are made by Causal-correlations and compounding, none is having Svabhava. Since all phenomena none is with Self-nature therefore it is empty, i.e., unobtainable (不可得). It is impermanent, conditioned. However, Nagarjuna didn't stop at here, not stopping at deconstructing, what he pointed to is, what beyond this Emptiness that given rise to all phenomena, the world and sentient... etc. Here is most of the scholastic Buddhist Masters/teachers fell. Such as, Tibetan Tsongkhapa by holding onto Madhyamika-Prasangika (wrong) viewpoint constructed his two Chenmo(s) misleading many Buddhists, bolting on the Intellect-mind (意識心), the 6th Vijñāna as permanent [which needed maintenance of each day 16 hours of xxx Tantric OgsM, all months, all years... well maybe good news for some, who knows!? ;] .
Nirvana and the Buddhahood are of the 5th Statement. The Intellect-mind is able to come as far as the Four-statements (Catuskoti) limitation. To go beyond the Intellect-mind, what should be looked for by all practitioners but the Self-nature as in Ch'an (6th Patriarch Huineng: How so the Self-nature, originally is self-sufficient (何期自性, 本自具足)), or the Tathagatagarbha, or the Alaya... the teachings only available in the Mahayana Canon (excluding the later Tantric texts). One of the most important Sutras is Prajna-paramita Sutra, which the related is Heart Sutra. I'm afraid Thich Nhat Hanh though a respectable teacher I respected, his interpretation of Heart Sutra as quoted in this post is questionable. It's only max. to the level of a Śrāvaka, not for the Bodhisattva or Buddha, i.e., not of the Prajna-paramita Heart Sutra the ultimate Emptiness.
The OP asked another question on usefulness of Madhyamaka, the key answer is, due to the decline of man's capacity, very rare few able to see directly the Ultimate Reality (still an ignorant claims able to see in meditation and, libeling Nagarjuna :), as stated in the beginning of the Madhyamaka Chinese Text. One of the purposes of Nagarjuna, I think, was enabling a glimpse of Ultimate Reality by pushing the Intellect-mind to the ultimate, by exhausting the Four-statements. If one able to read the original Classical Chinese one will at least sharpen his mind, have a shot of intelligence. Those who incessantly libeling the Mahayana Sutras, pity are they misled by their teachers or traditions. However, this very act is so detrimental to wisdom cultivation even to the worst severed the root of Prajna-paramita rendered these people the Icchantikas warned the Buddha. The Buddha explaint what is Icchantika and why they became the Icchantikas, one of the karmic deeds was they libeled Mahayana teaching due to their ignorance they lack the wisdom to understand the profound deep deep Mahayana doctrines, recorded in Prajna Paramita Sutra. There some so-called "Sutras" inserted, the later Tantric text, unfortunately, in the Tripitaka; but those great Sutras of Prajna-paramita - The 2nd Turning, also Tathagatagarbha - The 3rd Turning, are spoken by Buddha Shakyamuni.