OUSIA is ‘essence’ or ‘being’, but not exactly ‘substance’. This ancient Greek word is widely in use among past times philosophers like Platon (Plato) and Aristoteles (Aristotle), to indicate ‘essence’ or ‘being’. Later this term corresponds to the Latin words <essentia> or <substantia>.
In contemporary philosophy, the word <ousia> is analogous to English concepts like ‘being’ and ‘ontic’.
The issue of ‘being’ and ‘essence’ is one of the most engaging and challenging of philosophy. In order to explain the problem, let us talk about a color as red. When indicating a color, we designate it by association. For instance we say that a strawberry is red. A tomato is red. The blood is red. Ferrari’s color is red. But what is ‘red’ itself? We say that some things are red. But we do not say ‘what red is’ without qualifications. Hence, <ousia> is the answer to the problem of “what is being” when the question is without qualification. The unqualified answer of what is red is the <ousia> (‘essence’) of red.
Martin Heidegger says that the initial interpretation of the word <ousia> was lost in its translation to the Latin. As a consequence it was also lost in its translations to modern languages. For the German philosopher, <ousia> precisely means ‘being’, not ‘substance’, that is not some ‘thing’ or some ‘being’ that “stands” (-stance) “under” (sub-). Moreover, he uses the binomial <parousia>/<apousia>, denoting presence/absence, and <hypostasis> denoting ‘existence’.
<Ousia> is ‘essence’ or ‘being’, two very important and debated philosophical and theological concepts. In fact, the word is in use also among Christian theologians. The question is easy to grasp. If there is not an ‘essence’, beyond qualifications, it means that reality is constant variation and impermanence. For instance, a stone would only be the sum of different moments.