Wiktionary
n. A piece of furniture on which someone can pray, consisting of a cushioned area to kneel on, with a built-in ledge for books.
WordNet
n. low bench for kneeling on
Wikipedia
A prie-dieu ( French: literally, "pray [to] God", plural prie-dieux) is a type of prayer desk primarily intended for private devotional use, but may also be found in churches. It is a small, ornamental wooden desk furnished with a thin, sloping shelf for books or hands, and a kneeler. Sometimes, instead of the sloping shelf, a padded arm rest will be provided. This type is useful for devotions, such as the Rosary which do not require a book, or for private, non-liturgical prayer.
The prie-dieu appears not to have received its present name until the early 17th century. In that period in France, a small room or oratory was sometimes known by the same name. A similar form of chair in domestic furniture is called "prie-dieu" by analogy. Sometimes, a prie-dieu will consist only of the sloped shelf for books without the kneeler.
Prie-dieux may be provided in church weddings for the bride and groom to kneel on during the service, or may be used by a cleric when he leads the worshippers in prayers such as litanies.
In the Byzantine Rite, a prie-dieu is provided for the bishop when he kneels in the Holy Doors during the consecration of a church. One may also be used by the priest reciting Kneeling Prayers at Pentecost.