The term 'Royal Rule' can be summed up by the crossword solution 'REIGN, ' which is a five-letter word. REIGN is defined as the period of time during which a monarch rules a kingdom or empire. It is often used to refer to the rule of a single monarch, such as a king or queen. Royal Rule is the concept that a monarch has absolute power over their kingdom or empire, and is responsible for making laws and decisions that affect the people living in it.