A morganatic marriage is one in which one partner is of higher social rank, and the other cannot claim this partner's title or possessions or pass them to their children.
a morganatic marriage/union贵贱通婚/成婚
the Archduke Franz Ferdinand's morganatic wife, Sophie Chotek弗朗兹·斐迪南大公爵地位较低的妻子索菲·霍泰克