What is the correct term of address to the Pope

Your Holiness

His Holiness (Latin: Sanctitas) is the official style used to address the Roman Catholic Pope and the Greek Orthodox Ecumenical Patriarch.

In February 2013, the Holy See announced that Pope Benedict XVI will retain the style "His Holiness" after resigning and becoming Pope Emeritus.

The term is sometimes abbreviated or "HH" or "H.H." when confusion with "His/Her Highness" is unlikely. The associated form of address is "Your Holiness".

The English language honorific "His Holiness", and as female version "Her Holiness", has also been used for religious leaders from other traditions, including Shinto, Buddhism, Ahmadiyya, and it is used in certain new religious movements such as Hare Krishnas of Hinduism.

Read more...https://en.wikipedia.org/wiki/His_Holiness