baptistery

or baptistry

Domed hall or chapel, adjacent to or part of a church, for the administration of baptism. By the 4th century, the baptistery had assumed an eight-sided shape (eight in Christian numerology being the symbol for a new life), as had the baptismal font within. The font itself was set beneath a dome-shaped baldachin and encircled by columns and an ambulatory (aisle), features first used by the Byzantines.

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