lavabo
A ritual involving the washing of one's hands at a church's offertory before handling the Eucharist.
The small towel used to dry the priest's hands following the ritual.
These small liturgical towels got […] the name of Lavabo cloths or Lavaboes.
A washbasin, particularly (Christianity) the one in a church used in the ritual.
[…] the […] pontiff, as he […] moved his hands […] at the Lavabo, or at the various benedictions […]
A trough used for washing at some medieval monasteries.
Where it comes from
From the Christian ritual, which was traditionally accompanied in Catholicism by a recitation of Psalm 25:6 (Psalm 26:6, in the Hebrew and in most modern translations) which in Latin begins with the word lavābō (“I will wash”). The sense of "room with a toilet" developed under the influence of lavatory and lav.
Adapted from the Wiktionary entry lavabo (CC BY-SA 4.0).