Easton's Bible Dictionary
Kingdom of God
(Matthew 6:33; Mark 1:14, 15; Luke 4:43) = "kingdom of Christ" (Matthew 13:41; 20:21) = "kingdom of Christ and of God" (Ephesians 5:5) = "kingdom of David" (Mark 11:10) = "the kingdom" (Matthew 8:12; 13:19) = "kingdom of heaven" (Matthew 3:2; 4:17; 13:41), all denote the same thing under different aspects, viz.: (1) Christ's mediatorial authority, or his rule on the earth; (2) the blessings and advantages of all kinds that flow from this rule; (3) the subjects of this kingdom taken collectively, or the Church.
Noah Webster's New International Dictionary of the English Language
1. (n.) The rank, quality, state, or attributes of a king; royal authority; sovereign power; rule; dominion; monarchy.
2. (n.) The territory or country subject to a king or queen; the dominion of a monarch; the sphere in which one is king or has control.
3. (n.) An extensive scientific division distinguished by leading or ruling characteristics; a principal division; a department; as, the mineral kingdom.