The piscina is a Latin word originally applied to a fish-pond, and later used for natural or artificial pools for bathing, and also for a water tank or reservoir. In ecclesiastical usage it was applied to the basin used for ablutions and sometimes other sacraments.
They were originally named for the baptismal font. Piscinae seem at first to have been mere cups or small basins, supported on perforated stems, placed close to the wall, and afterwards to have been recessed therein and covered with niche heads, which often contained shelves to serve as ambries. They were rare in England until the 13th century, after which there is scarcely an altar without one. They frequently take the form of a double niche, with a shaft between the arched heads, which are often filled with elaborate tracery. If there is no drain, a niche for washing is a lavabo, though the usage of the two terms is confused.