Lavabo is Latin for "I will wash."   It refers both to the ritual of washing the priest's hands, as well as to the vessels used for this purpose.

It receives its name from the original Latin words of Psalm 26 "I wash my hands in innocence, and go around your altar, O Lord, singing aloud a song of thanksgiving, and telling all your wondrous deeds."

Formerly the priest would recite these words while the washing was happening, but this tradition has stopped.

In ancient times the ritual of lavabo was an obvious precaution for cleanness, but it came to be interpreted symbolically as a sing of inward purity.

In Eastern rites this ritual washing is part of vesting, which occurs before the service.

In the West it occurs after the offertory, before the priest begins to handle the gifts and vessels.

In High Anglican (Anglo-Catholic) parishes, this ritual occurs immediately after the priest has censed the altar, and been censed himself.

At which point a server steps forward with a bowl, over which the priest holds his fingers, a cruet or some other vessel containing water to be poured over the fingers, and a lavabo towel, to dry the priest's hands when the ritual washing is complete.

Saint Cyril of Jerusalem (4th century) mentions a ritual washing which takes place in front of the people, to sign purity of the soul, an idea which has been insisted upon by theologians such as St. Thomas Aquinas.

In modern times, one could consider the lavabo the forerunner of sinks, and any room in which a sink exists for the purpose of washing hands could rightly be called a Lavatory.