courtesy title

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A title of no legal validity that is assumed or granted by custom, such as the academic title professor given to any instructor at a college.
• noun In Great Britain, the title that the heir of a high-ranking peer customarily uses, consisting of a secondary title accorded to the peer.
• noun In Great Britain, the prefixes Lord and Lady added to the given names of the younger children of dukes and marquises or the Honourable added to the children of viscounts and barons.