Why is the First Church of Christ, Scientist called the Mother Church?

The First Church of Christ, Scientist, in Boston is known as the Mother Church because it serves as the central controlling body for the entire Christian Science movement.1

Established by Mary Baker Eddy, the church was reorganized in 1892 into a federal structure where all local Churches of Christ, Scientist, operate as branches of this central institution. The Mother Church provides the governing framework for the religion, with its laws, activities, and organizational structure detailed in the Manual of The Mother Church.12

Administration is overseen by a self-perpetuating Board of Directors, which manages the church's global activities and agencies, while the individual branch churches maintain a self-governing, democratic character within this overarching system.12