Confessor is a title used within Christianity in several ways.
Confessor of the Faith
Its oldest use is to indicate a saint who has suffered persecution and torture for the faith but not to the point of death. The term is still used that way in the East. In Latin Christianity it has come to signify any saint, as well as those who have been declared blessed, who can't be categorised by another title: martyr, apostle, evangelist, or virgin. As Christianity emerged as the dominant religion in Europe, persecutions became rare, and the title was given to saints who lived a holy life and died in peace. Perhaps the most well known example is the English king St. Edward the Confessor.
Confession of sins
During the Diocletianic Persecution, a number of Christians had, under torture or threat of torture, weakened in their profession of the faith. When the persecutions ceased under Constantine the Great, they wanted to be reunited with the Church. It became the practise of the penitents to go to the Confessors, those who had willingly suffered for the faith and survived, to plead their case and effect their restoration to communion. Thus, the word has come to denote any priest who has been granted the authority to hear confessions. This type of confessor might additionally be referred to as a "spiritual father." In the case of a monarch, the confessor might additionally fill the role of confidential and disinterested advisor.
In this sense of the term, it is standard practise for a religious community of women, either if enclosed or just quite large, to have a priest, to serve as confessor to the community, serving their spiritual needs.
It can additionally be used as the title of the head of a religious society.