baptism

Baptism is any water purification ritual practiced in many of various religions including Christianity, Mandaeanism, and Sikhism, and has its origins with the Jewish ritual of mikvah. The word baptize derives from the Greek word, which loosely means "to dip, bathe, or wash". To some groups it is a matter of religious conviction to assert that baptism is precisely equivalent to, to plunge something entirely into the water, so that the water closes over it.

http://en.wikipedia.org/wiki/Baptism