In pectore

(Latin for "in the breast/heart") is a term used in the Catholic Church for an action, decision, or document which is meant to be kept secret. It is most often used when there is a papal appointment to the College of Cardinals without a public announcement of the name of that cardinal. The pope reserves that name to himself. The Italian language version of the phrase – ''in petto'' – is sometimes used. When the name of a new cardinal is announced or made public, it is sometimes said to be ''published''.

Since the practice arose in the sixteenth century its use has varied greatly. Some popes have used it rarely or not at all, while others have used it regularly. In the first half of the 19th century, Pope Gregory XVI appointed half of his 75 cardinals ''in pectore'' and left several unidentified at his death. Provided by Wikipedia