Highness

Highness is a formal style used to address someone (in second person) or refer to (in third person) certain members of a reigning or formerly reigning dynasty. It is typically used with a possessive adjective: "His Highness", "Their Highnesses", etc. Although often combined with other adjectives of honour indicating rank, such as "Imperial", "Royal" or "Serene", it may be used alone.

Highness is, both literally and historically, the quality of being lofty or above, used as a term to evoke dignity or honour, and to acknowledge the exalted or official high rank of the person so described.

Abstract styles arose in profusion in the Roman Empire, especially in the Byzantine continuation. Styles were attached to various offices at court or in the state. In the early Middle Ages such styles, couched in the second or third person, were uncertain and much more arbitrary, and were more subject to the fancies of secretaries than in later times.

In English usage, the terms Highness, Grace and Majesty, were all used as honorific styles of kings, queens and princes of the blood until the time of James I of England. Thus in documents relating to the reign of Henry VIII of England, all three styles are used indiscriminately; an example is the king's judgment against Dr. Edward Crome (d. f562), quoted, from the Lord Chamberlains' books, ser. I, p. 791, in Trans. Roy. Hist. Soc. N.S. lOX. 299, where article 15 begins with Also the Kinges Highness hath ordered, 16 with Kinges Majestie, and 17 with Kinges Grace. In the Dedication of the Authorized Version of the Bible of 1611, James I is still styled Majesty and Highness; thus, in the first paragraph: "the appearance of Your Majesty, as of the Sun in his strength, instantly dispelled those supposed and surmised mists ... especially when we beheld the government established in Your Highness and Your hopeful Seed, by an undoubted title". It was, however, in James I's reign that Majesty became the official style.

...
Wikipedia