83 For there are today, Venerable Brethren, those who, approximating to errors long since condemned teach that in the New Testament by the word "priesthood" is meant only that priesthood which applies to all who have been baptized; and hold that the command by which Christ gave power to His apostles at the Last Supper to do what He Himself had done, applies directly to the entire Christian Church, and that thence, and thence only, arises the hierarchical priesthood. Hence they assert that the people are possessed of a true priestly power, while the priest only acts in virtue of an office committed to him by the community. Wherefore, they look on the eucharistic sacrifice as a "concelebration," in the literal meaning of that term, and consider it more fitting that priests should "concelebrate" with the people present than that they should offer the sacrifice privately when the people are absent.
Source: Mediator Dei (Vatican.va)