A short definition by Justo González

Priesthood of all believers

The principle that all Christian believers are priests by virtue of their baptism. This has its historical background in the Old Testament view of the people of God as "a kingdom of priests," and in its continuation in the New Testament. Historically, however, this universal priesthood was eclipsed by the place of the ordained priesthood in the "hierarchy of the church. It was Luther, and the Protestant Reformation with him, that made this once again a fundamental point of Christian doctrine - although Luther himself, and most of the Reformers, were unable to find ways to make such universal priesthood an experienced reality in the reality in the life of the church. Post- Vatican Roman Catholicism also recognizes the priesthood of all believers, although still insisting that the difference between this priesthood and that of the ordained clergy is more than a mere matter of degree. (Essential Theological Terms By Justo L. González)