Monseigneur is an honorific in the French language. It has occasional English use as well, as it may be a title before the name of a French prelate, a member of a royal family or other dignitary.
Monsignor is both a title and an honorific in the Roman Catholic Church. In francophone countries, it is rendered Monseigneur, and this spelling is also commonly encountered in Canadian English practice.
In France, monsignori are not usually addressed as monseigneur, but by the more common term monsieur l'abbé, as are priests. The plural form is Messeigneurs.
Prior to the overthrow of the French monarchy in 1792, Monseigneur equated to His Royal Highness or His Serene Highness when used as part of the title of a royal prince, as in Monseigneur le comte de Provence. King Louis XIV promoted the use of Monseigneur without the title as a style for the dauphin de France but this use lapsed in the 18th century. French royalists commonly style the current pretender Monseigneur. In his book A Tale of Two Cities Charles Dickens uses this honorific as a collective noun denoting the great nobility as a class.