Easton's Bible Dictionary
An inflammable mineral substance found in quantities on the shores of the Dead Sea. The cities of the plain were destroyed by a rain of fire and brimstone (Genesis 19:24, 25). In Isaiah 34:9 allusion is made to the destruction of these cities. This word figuratively denotes destruction or punishment (Job 18:15; Isaiah 30:33; 34:9; Psalm 11:6; Ezek. 38:22). It is used to express the idea of excruciating torment in Revelation 14:10; 19:20; 20:10.
Noah Webster's New International Dictionary of the English Language
1. (n.) Sulphur; also passionate, fiery rhetoric or preaching.
2. (a.) Made of, or pertaining to, brimstone; as, brimstone matches.