mother superior
Synonym discussion
Sense: noun/1
All three name a woman who leads a community of nuns, but they differ in formality and rank. 'Mother superior' is the everyday term and works as a respectful title you can use directly, as in 'Yes, Mother Superior.' 'Abbess' is the formal title for the head of an abbey, a larger and more independent house, and carries more authority within the wider order. 'Prioress' usually names the head of a priory, a smaller house, and in some orders ranks just below an abbess. In casual speech most people say mother superior; in church documents or histories you are more likely to read abbess or prioress. For example, a newspaper might call her 'the mother superior of the local convent', while a formal record would list her as 'the abbess of St Hilda's'.
Etymology
The phrase joins 'mother', long used as a respectful term for a senior nun, with 'superior', from Latin 'superior' meaning 'higher in rank'. In religious houses 'superior' came to name the person placed above the others, so a 'mother superior' is literally the female head set over the rest of the community.