Related to Latin regere (to guide, keep straight) is another Latin root rex (or reg-) meaning king. A regal stride or stance is thus ‘king-like’ and regalia literally means (rights and privileges) of the king or queen. Later, regalia also began to be used to refer to the crown, emblems, ornaments etc used at the coronation, from which it gets its sense of elaborate clothes or decorations, as in, the troops marched in full military regalia or he looked distinguished attired in ceremonial regalia.
A related (but now rarely used) word is French regale which referred to specific rights of the king of France. The other sense of the word, as used in phrases such as ‘he regaled (entertained) her with his antics’ and ‘they regaled (provided with choice of food and drink) their guests’ comes from French gale (rejoicing, merriment) and galer (make merry).
Another related Latin root regnum (kingship or rule) became reigne in Old French and reign in Eng (as in, he unleashed a reign of terror). And if we combine inter and regnum we get interregnum which literally means between rules and refers to a gap or duration in governance or rule.
Regnant literally means ruling and can be used more broadly to mean predominant or widespread (for example: contrary to the regnant belief, the world seems to be getting less violent with every passing century). A regent, on the other hand, is a person who has the authority to act (govern, rule) on behalf of royalty (for instance if the heir to the throne is too young to rule).
High Frequency Words for GRE, GMAT, SAT, CAT & Other Admission Tests*:
Regal*, Regalia*, Regale**
Reign, Interregnum**, Royal, Regnant*, Regent