What is the meaning of Regnum?

A rank in the classification of organisms, below dominium and above divisio.

A badge of royalty, especially the early form of the pope's tiara.

royal power, power, control, reign

kingdom, realm, throne

kingship, royalty

in a negative sense: despotism, tyranny, aspiring to or seeking the throne or royal power during the Roman Republic

Source: wiktionary.org