From the same as charax; a graver (the tool or the person), i.e. (by implication) engraving (("character"), the figure stamped, i.e. An exact copy or (figuratively) representation) -- express image.
KJV: Who being the brightness of his glory, and the express image of his person, and upholding all things by the word of his power, when he had by himself purged our sins, sat down on the right hand of the Majesty on high;
NASB: And He is the radiance of His glory and the exact representation of His nature, and upholds all things by the word of His power. When He had made purification of sins, He sat down at the right hand of the Majesty on high,