Other forms: abbesses
An abbess is the head of a group of nuns. Typically, a woman has been a nun for many years before becoming an abbess.
In the Catholic church, the male superior of monks is called an abbot. The female equivalent is an abbess, who's kind of the boss of the nuns at an abbey (which simply means "place where monks or nuns live"). We can trace this word all the way back to the Aramaic abba, an honorific title that means "the father" or "my father."