A "royal duke" is an important title given to members of the British royal family who hold the rank of a duke. However, there are several other synonyms used to refer to these distinguished individuals. One commonly used term is "prince," an honorary title bestowed upon those with royal lineage. Another synonym is "archduke," a title given to those who hold a high-ranking position in certain European monarchies. The term "nobleman" can also be used to describe a royal duke, as they are part of the highest echelons of aristocracy. Lastly, the term "peer" can also be used to describe a royal duke, as they are part of the House of Lords in the British government.