Easton's Bible Dictionary

Sea of glass - a figurative expression used in Rev. 4:6 and 15:2. According to the interpretation of some, "this calm, glass-like sea, which is never in storm, but only interfused with flame, represents the counsels of God, those purposes of righteousness and love which are often fathomless but never obscure, always the same, though sometimes glowing with holy anger." (Comp. Ps. 36:6; 77:19; Rom. 11:33-36.)