That which forms the basis of anything; underlying principle; a concept or mental entity conceived or treated as an existing being or thing.
Principle; an element; -- used by the alchemists in speaking of salt, sulphur, and mercury, which they considered as the three principles of all material bodies.
(metaphysics) essential nature or underlying reality
(as defined here, in the Plotinian sense) underlying Divine or Absolute Reality, the Truth or Foundation or Essence or Support or Cause of a thing more
an underlying substance, as opposed to an attribute or that which is insubstantial
The word hypostasis means underlying state or underlying substance. It is the fundamental reality that supports all else. Neoplatonists argue that behind the surface phenomena that present themselves to our senses are three higher spiritual principles or hypostases, each one more sublime than the preceding.
In linguistics, a hypostasis, from the Greek word ὑπόστασις http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%23109195 meaning foundation, base or that which stands behind is a relationship between a name and a known quantity, as a cultural personification (i.e. objectification with personality) of an entity or quality. It often connotes the personification of typically elemental powers, such as wind and fire, or human life, fertility, and death.
In Christian usage, the Greek word hypostasis , the range of whose meanings is illustrated in http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus:text:1999.04.0057:entry=%23109195 Liddel and Scott's Greek Lexicon, has a complicated and sometimes confusing history. Its basic meaning is "that which stands beneath". It was used by, for instance, Aristotle and the Neoplatonists, to speak of the objective reality (as opposed to outer form or illusion) of a thing, its inner reality.