What is Sacristan?

Legal Definition
A sacristan is an officer charged with care of the sacristy, the church, and their contents.

In ancient times, many duties of the sacristan were performed by the doorkeepers (ostiarii), and later by the treasurers and mansionarii. The Decretals of Gregory IX speak of the sacristan as if he had an honourable office attached to a certain benefice, and say that his duty was to care for the sacred vessels, vestments, lights, etc. Nowadays the sacristan is elected or appointed. The Cæremoniale Episcoporum prescribed that in cathedral and collegiate churches the sacristan should be a priest, and describes his duties in regard to the sacristy, the Blessed Eucharist, the baptismal font, the holy oils, the sacred relics, the decoration of the church for the different seasons and feasts, the preparation of what is necessary for the various ceremonies, the pregustation in pontifical Mass, the ringing of the church bells, the preservation of order in the church, and the distribution of Masses; and finally it suggests that one or two canons be appointed each year to supervise the work of the sacristan and his assistants.
-- Wikipedia
Legal Definition
A sexton, anciently called "sagerson," or "sagiston;" the keepor of things belonging to divine worship.
-- Black's Law Dictionary
Legal Definition
A sexton.
-- Ballentine's Law Dictionary