Meaning

noun a domain in which something is dominant

noun the domain ruled by a king or queen

noun a knowledge domain that you are interested in or are communicating about

originFrom Middle English rewme, realme, reaume, from Old French reaume, realme, reialme (“kingdom”), of unclear origins. A postulated *rēgālimen (“domain, kingdom”), Late Latin or Vulgar Latin cross of regimen with rēgālis is usually cited. The modern spelling predominates from around 1600. The modern pronunciation with /l/ is either a spelling pronunciation or influenced by the etymology.

Etymology from Wiktionary, CC BY-SA 3.0