noun

1. The Church regarded as nourishing and protecting its members.

   “Catholics regard Mary as Mother of the Church but an all-male hierarchy as the Mother Church.”

2. A church with oversight over another or others, now especially a cathedral or metropolitan church.

   “Mothering Sunday arose from the custom of visiting mother churches at Mid-Lent, rather than the usual parish church.”

3. The original church of a denomination, regarded as having birthed the others.

   “The mother church of Christian Scientists is in Boston, Massachusetts.”