Ceremonial ablution, part of Islamic ritual.
Having sexual intercourse; menstruating; giving birth; or having physical contact with a corpse renders a Muslim ritually unclean. He or she must undergo ghusl before being allowed to participate in prayer.
The process of performing ghusl involves a full immersion of the body (which differentiates it from wudu). All parts of the body should be rinsed, including every hair on the head, and the nose and mouth.
Thanks to robwicks for pointing out that this WU needed to be expanded to clarify the difference between ghusl and wudu.