means A country ruled by a king or queen, or more broadly any distinct realm, domain, or category over which something holds sway.

from From Old English 'cyningdom' — 'cyning' (king) plus the suffix '-dom,' which means 'state, condition, or domain of.' That same '-dom' lives on in 'freedom,' 'wisdom,' and 'boredom,' all of them little territories of being. 'Cyning' itself likely descends from a Proto-Germanic root tied to 'kin' — the king as the man of the kindred, the head of the family writ large. So a kingdom is, at its bones, the condition of having a kin-leader. Later it stretched far beyond crowns: by the time naturalists were sorting life into the animal, plant, and mineral kingdoms, the word simply meant a vast realm of its own kind.