From John Piper's sermon dated January 1, 1984
"The glory of God is the manifestation of his holiness. God's holiness is the incomparable perfection of his divine nature; his glory is the display of that holiness. "God is glorious" means: God's holiness has gone public. His glory is the open revelation of the secret of his holiness. In Leviticus 10:3 God says, "I will show myself holy among those who are near me, and before all the people I will be glorified." When God shows himself to be holy, what we see is glory. The holiness of God is his concealed glory. The glory of God is his revealed holiness."
From John Piper's sermon dated May 26, 2003
"God's holiness is his infinite value as the absolutely unique, morally perfect, permanent person that he is, who by grace made himself accessible in Jesus Christ."
(HT: Todd Leroy in response to a comment on Justin Taylor's blog.)
(UPDATE: Al Mohler's definition of the glory of God from a talk he gave at Capitol Hill Baptist Church: "The intrinsic reality and the outward manifestation of God's power and character.")