What is the female equivalent of a marquess in British nobility?

A marquess is a nobleman of high hereditary rank in various European peerages and in those of some of their former colonies. A marchioness is a noblewoman with the rank of marquess, or the wife of a marquess. The honorific prefix "The Most Honourable" precedes the name of a marquess or marchioness of the United Kingdom.

There has only been one woman created a marchioness in her own right:

Anne Boleyn (c1501-1536) – in preparation for her wedding to King Henry VIII, she was created Marchioness of Pembroke in a ceremony held at Windsor Castle in 1532. The couple married several months later, and Anne was Queen of England until her beheading in 1536. The title was created with remainder to her “heirs male”, making it the first hereditary peerage granted to a woman.

In Great Britain and historically in Ireland, the correct spelling of the aristocratic title of this rank is "marquess" (although on the European mainland and in Canada, the French spelling "marquis" is used). In Scotland the French spelling is also sometimes used. In Great Britain and historically in Ireland, the title ranks below a duke and above an earl. A woman with the rank of a marquess, or the wife of a marquess, is called a marchioness in Great Britain and Ireland, or a marquise elsewhere in Europe. A marchioness ranks below a duchess and above a countess.

The ranks of marquess and marchioness were a relatively late introduction to the British peerage.