a title applied or assumed through custom, courtesy, or association and without regard for its being officially merited.
a title allowed by custom, as to the children of dukes.
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How to use courtesy title in a sentence
The term became in time a mere courtesy title but originally carried with it standing.
Just as the eldest son of a Duke bears a "courtesy title," so does the eldest son of a Marquis.Book of Etiquette, Volume 2 | Lillian Eichler Watson
In the United States navy “commodore” was a courtesy title given to captains who had been in command of a squadron.
It is also given as a courtesy title to the senior officer of a squadron of more than three vessels.
I have called you my 'wife,' I know; but it has just been a courtesy title.The Following of the Star | Florence L. Barclay
British Dictionary definitions for courtesy title
any of several titles having no legal significance, such as those borne by the children of peers
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012