An abbot was a religious title, in Christianity usually reserved for the leader of a group of monks.
While speaking with Catarina about seeking imagination, Leonardo da Vinci asked her to accompany him to the monks of Santa Croce where he was supposed to deliver a bronzetto. He suggested to wake the abbot, visit the local chapel and appeal to god. (VOY: "Scorpion")
The term was also used by the Kadi, with the Abbot conducting diplomatic talks with the USS Voyager in 2375. He was addressed as "His Holiness". (VOY: "Someone to Watch Over Me")