Easton's Bible Dictionary
The plague (the ninth) of darkness in Egypt (Exodus 10:21) is described as darkness "which may be felt." It covered "all the land of Egypt," so that "they saw not one another." It did not extend to the land of Goshen (ver. 23).
On Mount Sinai, Moses (Exodus 20:21) "drew near unto the thick darkness where God was." This was the "thick cloud upon the mount" in which Jehovah was when he spake unto Moses there. The Lord dwelt in the cloud upon the mercy-seat (1 Kings 8:12), the cloud of glory. When the psalmist (Psalm 97:2) describes the inscrutable nature of God's workings among the sons of men, he says, "Clouds and darkness are round about him." God dwells in thick darkness.
Darkness (Isaiah 13:9, 10; Matthew 24:29) also is a symbol of the judgments that attend on the coming of the Lord. It is a symbol of misery and adversity (Job 18:6; Psalm 107:10; Isaiah 8:22; Ezek. 30:18). The "day of darkness" in Joel 2:2, caused by clouds of locusts, is a symbol of the obscurity which overhangs all divine proceedings. "Works of darkness" are impure actions (Ephesians 5:11). "Outer darkness" refers to the darkness of the streets in the East, which are never lighted up by any public or private lamps after nightfall, in contrast with the blaze of cheerful light in the house. It is also a symbol of ignorance (Isaiah 9:2; 60:2; Matthew 6:23) and of death (Job 10:21; 17:13).
Noah Webster's New International Dictionary of the English Language
1. (n.) The absence of light; blackness; obscurity; gloom.
2. (n.) A state of privacy; secrecy.
3. (n.) A state of ignorance or error, especially on moral or religious subjects; hence, wickedness; impurity.
4. (n.) Want of clearness or perspicuity; obscurity; as, the darkness of a subject, or of a discussion.
5. (n.) A state of distress or trouble.