A courtesy title is a title that does not have legal significance but is rather used by custom or courtesy, particularly, in the context of nobility, the titles used by children of members of the nobility (cf. substantive title).[1][2]
In some contexts, courtesy title is used to mean the more general concept of a title or honorific such as Mr., Mrs., Ms., Miss, Madam, Sir for those who not been awarded a knighthood or a baronetcy, as well as Dr. for physicians who have not actually achieved a doctorate.[3]