regnum

Regnum, regni, n., second declension, built straight off rex, regis with the abstract suffix -num. It carries two senses that English splits between "kingdom" and "reign": the territory ruled, and the royal power itself.

Under the Republic the second sense was radioactive. Regnum adfectare, to aim at kingship, was an accusation that killed men, and the historians use the word about Roman politics only as a charge. It becomes neutral once there are kings again to describe, and Christian Latin makes it ordinary: regnum caelorum, the kingdom of heaven, and adveniat regnum tuum in the Lord's Prayer. The verb is regno, "I reign", with the person ruled in the dative or governed by a preposition rather than in the accusative. English takes reign, regnal and interregnum.