A black garment reaching down to the ankles; worn by priests or choristers WordNet 3.0 & Open English WordNet

The cassock or soutane is a Christian clerical clothing coat used by the clergy and male religious of the Oriental Orthodox Churches, Eastern Orthodox Church and the Catholic Church, in addition to some clergy in certain Protestant denominations such as Anglicans and Lutherans. Wikipedia

Ankle-length garment worn as Christian clerical clothing Wikidata

Long robe worn by clergymen. OmegaWiki

An item of clerical clothing: a long, sheath-like, close-fitting, ankle-length robe worn by clergy members of some Christian denominations. Wiktionary

Item of clerical clothing. Wiktionary (translation)