This word was known in classical Greek. It is composed of two parts: hypo meaning “under” and stasis meaning “setting.” In theology, it developed to mean reality, substance, nature, or essence. This word occurs 20 times in the Septuagint but only one of them can be regarded as theologically significant.
At the beginning of the fourth century the word hypostasis and the word ousia had pretty much the same meaning. St. Cyril denoted the hypostatic union, that is, the union of two distinct levels of reality, being divine and human in Christ, while Nestorius understood the word as a “physical connotation.” As for the Antiochene, in the time of St. Cyril this term had only a concrete meaning, hence “physis.” The diversion between the two conceptions led to the schism and the misunderstanding in the Council of Chalcedon.