The Collaborative International Dictionary of English v.0.48:

Hypostasis \Hy*pos"ta*sis\, n.; pl. Hypostases. [L., fr. Gr. ?
   subsistence, substance, fr. ? to stand under; ? under + ? to
   stand, middle voice of ? to cause to stand. See Hypo-, and
   Stand.]
   1. That which forms the basis of anything; underlying
      principle; a concept or mental entity conceived or treated
      as an existing being or thing.
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   2. (Theol.) Substance; subsistence; essence; person;
      personality; -- used by the early theologians to denote
      any one of the three subdivisions of the Godhead, the
      Father, Son, and Holy Spirit.
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   Note: The Council of Alexandria (a. d. 362) defined
         hypostasis as synonymous with person. --Schaff-Herzog.
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   3. 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.
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   4. (Med.) That which is deposited at the bottom of a fluid;
      sediment.
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