the shining of a glory: the brightness that belongs to and comes from a source of light
This word appears only once in the New Testament, in the opening of Hebrews, where the Son is called the brightness of God's glory. It pictures light and its shining: just as a source of light cannot be without its brightness, the Son is the very shining-out of who God is. The image is deliberately tight. There is no moment when the sun exists without its light, and the word is chosen to say the same of the Son and the Father.
The word is rare and learned. It does not turn up in everyday Greek; it belongs to a thoughtful, religious vocabulary, and its most famous earlier use is in the Jewish book the Wisdom of Solomon, where Wisdom is called the brightness of eternal light. The writer of Hebrews almost certainly has that line in mind and applies to the Son what that tradition had said of God's Wisdom.
There is a long-running question about the exact angle of the picture, captured in how translations differ. Some read it as light streaming out from its source (radiance, effulgence); others read it as light thrown back from a surface (reflection). The first reading has the stronger case and is now the usual one, but a number of careful readers think the word does not force the choice and that both ideas are in view: the Son both shines out from the Father and perfectly shows him. That debate is laid out below.