a long sleeveless vestment worn by a priest when celebrating Mass
source: WordNet 3.0
The outer vestment worn by the priest in saying Mass, consisting, in the Roman Catholic Church, of a broad, flat, back piece, and a narrower front piece, the two connected over the shoulders only. The back has usually a large cross, the front an upright bar or pillar, designed to be emblematical of Christ's sufferings. In the Greek Church the chasuble is a large round mantle.