A more thorough explanation:

Definition: A marchioness is a female noble title that is equivalent to a marquis. It can be obtained through marriage with a marquis or by creation.

For example, Catherine Middleton became a marchioness when she married Prince William, who is also known as the Duke of Cambridge. Her official title is "Her Royal Highness The Duchess of Cambridge, Countess of Strathearn, and Lady Carrickfergus," but she is also referred to as "Kate, the Marchioness of Cambridge."

Another example is the character Lady Mary Crawley from the TV show Downton Abbey. She becomes a marchioness when she marries Henry Talbot, who is later made a marquess.

These examples illustrate how a marchioness is a female noble title that is obtained through marriage or creation and is equivalent to a marquis.