Longman Dictionary of Contemporary English
Wikipedia
Grand Master (; ) is a title of the supreme head of various orders, including chivalric orders such as military orders and dynastic orders of knighthood.
In the Sovereign Military Order of Malta, the Grand Master is styled "Sovereign", e.g. Sovereign Grand Master, due to its status as an internationally independent sovereign entity.
As most present-day orders are essentially honorary distinctions - as some always were, e.g. the Order of the Golden Fleece - so are its dignities, which may be held by right of birth or another rank. Thus, a sovereign monarch often holds the title of Grand Master of the highest honorary dynastic orders of knighthood, or by custom awards it to a Prince of the royal family, regularly the heir to the throne, who in other orders may hold another high rank/title.
The title also occurs in modern civil fraternal orders such as the Freemasons, the Odd Fellows, and various other fraternities. Additionally, numerous modern self-styled orders attempt to imitate habits of the former bodies.
A Grand Master is a title of honour as well as an office in Freemasonry, given to a freemason elected to oversee a Masonic jurisdiction, derived from the office of Grand Masters in chivalric orders. He presides over a Grand Lodge, and has certain rights in the constituent Lodges that form his jurisdiction. In most, but not all cases, the Grand Master is styled "Most Worshipful Grand Master." One example of a differing title exists in the Grand Lodge of Pennsylvania, where the Grand Master is titled "Right Worshipful".