Mikveh מקווה
A pool of natural water effecting the passage from ritual impurity to purity.
A mikveh is not washing but a change of status. Its requirements are exact: at least forty se'ah of water that arrived naturally — rain, spring or melted snow — and was never carried in a vessel. Immersion must be total, with nothing intervening between body and water.
In Temple times it touched nearly everyone: without it one could not eat of the offerings, enter the Temple Mount or serve as a kohen. After the destruction the central area became the laws of family purity, along with immersing vessels bought from a non-Jew and immersion at conversion, one of the three conditions of joining the people.
Communities built a mikveh before a synagogue. Archaeologists find them by the dozen at Masada, at Qumran and beside the Temple Mount steps.