Easton's Bible Dictionary (1897)
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 (Gen. 19:24, 25). In Isa. 34:9 allusion is made to the destruction of these cities. This word figuratively denotes destruction or punishment (Job 18:15; Isa. 30:33; 34:9; Ps. 11:6; Ezek. 38:22). It is used to express the idea of excruciating torment in Rev. 14:10; 19:20; 20:10.
Smith's Bible Dictionary (1863)
Brimstone, or sulphur, is found in considerable quantities on the shores of the Dead Sea. (Genesis 19:24) It is a well-known simple mineral substance, crystalline, easily melted, very inflammable, and when burning emits a peculiar suffocating odor. It is found in great abundance near volcanoes. The soil around Sodom and Gomorrah abounded in sulphur and bitumen.
Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
BRIM'STONE . Ps 11:6. Sulphur, a well-known mineral substance, exceedingly inflammable, and which when burning emits a suffocating smell. We are told that the cities of the plain were destroyed by a rain of fire and brimstone. There is nothing incredible in this, even if we suppose only natural agencies were employed. Like many other travellers, the writer has pieces of pure sulphur and of asphalt or mineral pitch, both found in that vicinity in abundance and highly inflammable.
Volcanic action might easily have filled the air with inflammable substances, falling down in streams of liquid fire upon those devoted cities. This word is often figuratively employed. Job 18:15; Isa 34:9; Rev 21:8. Whether the word is used literally or not in the passages which describe the future sufferings of the wicked, we may be sure that it expresses terrible punishment.