1.NounThe monastery from which the other 'houses' of a religious order or congregation were (directly or indirectly) founded, often eponymous.
"The Cistercian order is named after its motherhouse Cîteaux in Burgundy, where abbot St. Bernard initiated in 1113 the reform of their branch of the Benedictine order, which has its motherhouse at Monte Cassino."
2.NounThe convent which is the seat (and often the above original foundation) of the superior of an order or congregation, and/or on which lower ranking houses (such as priories under an abbot) depend.