Of or pertaining to a form of marriage in which a person of high rank, as a member of the nobility, marries someone of lower station with the stipulation that neither the low-ranking spouse nor their children, if any, will have any claim to the titles or entailed property of the high-ranking partner.
Plans for a morganatic marriage, for Mrs Simpson to become Duchess of Cornwall, came to nothing. It is a mark of how much has changed in the intervening three-quarters of a century that the British press maintained a loyal silence about the crisis until the last moment, though the King's affair with a married woman was common knowledge among the upper classes.
-- Adam Sisman, "The Last Dance: 1936 by Denys Blakeway: review," The Telegraph, June, 2010.
Your marriage is therefore no marriage at all, and the highest compliment we can pay your association is to call it morganatic.
-- Robertson Davies, Hunting Stuart: & The voice of the people : two plays
Morganatic relates to the Latin morganaticam, "a gift given by bride or groom the day after marriage."