The term 'head nun' is a job description for an ABBESS, which is a female religious leader in charge of a convent or monastery. An ABBESS is typically responsible for the spiritual and administrative duties of the convent or monastery, such as providing spiritual guidance to the nuns, managing the finances, and overseeing the day-to-day operations. The ABBESS is also responsible for ensuring that the nuns adhere to the rules and regulations of the convent or monastery.