In the Episcopal Church, someone who is trained and licensed by the bishop of the Diocese to serve communion wine to parishioners during a church service.
The general idea is to increase the throughput of the practical matter of Communion by freeing the priests to dispense bread.
To become one, especially a good one, one must not only know the proper manner in which to administer the wine and clean the side of the chalice with the purificator, but also how your particular church has its ministers walk from parishioner to parishioner (sometimes they are double stacked and move not from one to the next, but one to one two over, coming back around in a loop after a certain distance).
They usually wear a floor length robe (ours are purple), and actually genuinely do have a physical license from the presiding Bishop. It's a nice slip of paper which is in practice never asked for. (The people scheduling the services know who has proper authority and who doesn't.)