A paten or diskos is a small plate, used during the Mass. It is generally used during the liturgy itself, while the reserved sacrament are stored in the tabernacle in a ciborium.
In many Western liturgical denominations, the paten is typically either a simple saucer-like plate or a low bowl. A smaller style paten will often have a depression that allows it to securely sit on top of the chalice, as shown in the illustration on the left here.
The General Instruction of the Roman Missal lays down rules for patens: "Sacred vessels should be made from precious metal. If they are made from metal that rusts or from a metal less precious than gold, they should generally be gilded on the inside." However, provisions for vessels made from non-precious metals are made as well, provided they are "made from other solid materials which in the common estimation in each region are considered precious or noble."
Some call the communion-plate a "paten", but the English translation of the General Instruction of the Roman Missal approved by the English-speaking episcopal conferences and confirmed by the Holy See uses "communion-plate" to speak of this object (numbers 118 and 187) and reserves the term "paten" for the other object (numbers 84, 117, 141, etc.). The two corresponding Latin terms are patina (communion-plate) and patena (paten).
Patens are also used among Anglicans and Lutherans.
In the United Methodist Church, during the Order for the Ordination of Elders, each elder receives a stole, along with a chalice and paten, from the bishop after the part of the liturgy in which the bishop lays his hands and prays over the ministerial candidates. This is because the newly ordained elders are now able to celebrate the Sacraments, such as Holy Communion. In the Methodist service of the Holy Communion, the bread is placed upon a paten during the offertory and once again after it consecrated, specifically following the fraction. The paten, along with the chalice, lies on the altar during the celebration of the Holy Eucharist.