The region, sphere, or domain within which anything occurs, prevails, or dominates
ξ bio-realms are distinct biological regions with unique ecosystems and species assemblages

»Realm by Merriam Webster
ξ Kingdom, Sphere, Domain

»Realm by Wikipedia
ξ the dominion of a monarch, king or queen, emperor or empress, or another personal sovereign ruler of some sort
ξ derives from medieval Latin regalimen, from regalis, of or belonging to a rex, (king)
ξ directly translates into reich in German, though the word reich is often used as a short form for 'kingdom' (Königreich) and especially 'empire' (Kaiserreich)
ξ German -reich is only used for realms headed by a crowned monarch