The phelonion (plural, phelonia) is a liturgical vestment worn by a priest over his other vestments. It was originally a sort of poncho, with a round hole in the middle for the head, and falling to the feet. In its present form, the front is largely cut away (from about the waist down) to facilitate the priest's movements. The use of the phelonion is not limited to the Divine Liturgy but is specified for any major liturgical function.
There are two main styles of phelonion. Byzantine or Greek phelonia are tailored to fit over the shoulders, while Russian phelonia have a high, stiffened collar that covers the back of the head. There is also a shortened phelonion that is worn by a reader at his tonsuring.
A bishop who wishes to serve a Divine Liturgy as a priest (i.e., without the special rites and prayers of the Hierarchical Divine Liturgy) will sometimes vest in a phelonion instead of his sakkos, but with the small omophorion around his neck.