An oratory is a Christian room for prayer, from the Latin orare, to pray.
In the Roman Catholic Church, an oratory is a structure other than a parish church, set aside by ecclesiastical authority for prayer and the celebration of Mass. It is for all intents and purposes another word for what is commonly called a chapel, except a few oratories are set up for the Divine Office and prayers but not Mass.
Previously, canon law distinguished several types of oratories: private (with use restricted to an individual, such as a bishop, or group, such as a family, and their invited guests), semi-public (open under certain circumstances to the public), or public (built for the benefit of any of the faithful who wish to use it). (Code of Canon Law, canon 1223). The term is used for instance in the Rule of St Benedict (chapter 52) for the private communal chapel inside monasteries.
The distinctions between public, semi-public, and private have been eliminated in the 1983 Code of Canon Law in favor of new terminology. Oratory now means a private place of worship for a group or community which could be opened to the public at the discretion of the group's superior. This definition corresponds with the semi-public oratory of the 1917 Code of Canon Law. The private oratory of the 1917 Code corresponds very closely with the 1983 Code's chapel, as they are both places of worship for specific individuals.
Oratories seem to have found their origin in chapels built at the shrines of martyrs, for the faithful to assemble and pray on the spot. The oldest extant oratory is the Archiepiscopal Chapel in Ravenna (c. 500). The term is often used of very small structures surviving from the first millennium, especially in areas where the monasticism of Celtic Christianity was dominant; in these cases it may represent an archaeological guess as to function, in the absence of better evidence.