Florence Harding was called the duchess by her husband.
She would be a Lady. She would be addressed as "Your Ladyship," unless she is a duchess, in which case she is called, "Your Grace." If her husband is a baron, she would be a baroness. If her husband is a count or earl, she would be a countess. If her husband is a marquess, she would be a marchioness; if he is a marquis, she is a marquise, and if he is a margrave, she is a margravine. If her husband is a duke, she is a duchess. There is more information at the link below.
The territory of a duke or duchess was called a Duchy.
Is it a Duch?
A male Duchess is called a DukeWait a minute. A male duchess would be called a transvestite, or perhaps more accurately transgendered.
Her husband. And she is no longer Kate Middleton but Catherine, Duchess of Cambridge.