υποστασεως (hupostaseos) means essence (Septuagint NT Greek Interlinear) or substance (Strong's concordance).
"who being the radiance of the glory, and impression of his essence..." (Heb.1:3 part, Septuagint NT Greek Interlinear).
ὑπόστασις (hupostasis) is that which underlies the apparent and which therefore is the reality, the essence or the substance. It came to denote essence, substance or the inner nature and as discussed below is used with that meaning here in Hebrews 1:3. The author is conveying the truth that whatever the divine essence is, Jesus is said to be its perfect expression and in so doing affirms the deity of Jesus Christ. The etymological equivalent of hupostasis in English is substance or that which stands under a thing and which makes it what it is. The Son is such a revelation of the Father that when we see Jesus, we see what God's real being is. [From[More at Precept Austin and Department of Christian Defense.]