The Greek noun hypostasis means substance, underlying reality, or confident assurance. Composed of hypo (under) and histemi (to stand), it literally means 'that which stands under' — the foundation, the substance, the underlying reality. It appears about 5 times in the New Testament.
Hypostasis is one of the most philosophically and theologically loaded words in the New Testament and in the history of Christian doctrine. In Hebrews 11:1, it defines faith itself: 'Now faith is the hypostasis of things hoped for, the evidence of things not seen.' The word means faith is not wishful thinking but the underlying substance — the title deed, the guarantee — of what God has promised. Faith treats future realities as present because God's word makes them real. In Hebrews 1:3, it describes the Son's relationship to the Father: the Son is 'the exact representation (charakter) of his [the Father's] hypostasis' — His very substance, His essential nature. This verse became foundational for the Nicene theology of Christ's full divinity: He is not a copy or emanation but the precise imprint of God's own being. In later Trinitarian theology, hypostasis became the technical term for each Person of the Trinity — Father, Son, and Spirit are three hypostaseis in one divine essence (ousia). This technical use flows directly from Hebrews 1:3.