An abbot (the word derives from Abba, meaning "father") is, generally, the head of a monastery, although the title was also applied to other posts, mostly of the ecclesiastical persuasion.
It originated in Egyptian monasticism, either as the head of one community or of several. By the Rule of St. Benedict, which was the monastic norm in the Western Church, he governed one monastery. It also became the norm for them to be in religious orders, a deacon at least, if not a fully-ordained priest. They were initially subject to episcopal oversight but little by little the Popes allowed them greater authority, until each abbot became vested with authority equal to a bishop and in some cases an archbishop, and eventually no less than 24 English abbots were entitled to wear the mitre and ring as if bishops.
Generally, an Abbot is properly addressed simply as "Father" or "Father Abbot".