60 And now, Venerable Brethren, We come to that part of Our explanation in which We desire to make clear why the Body of Christ, which is the Church, should be called mystical. This name, which is used by many early writers, has the sanction of numerous Pontifical documents. There are several reasons why it should be used; for by it we may distinguish the Body of the Church, which is a Society whose Head and Ruler is Christ, from His physical Body, which, born of the Virgin Mother of God, now sits at the right hand of the Father and is hidden under the Eucharistic veils; and, that which is of greater importance in view of modern errors, this name enables us to distin guish it from any other body, whether in the physical or the moral order.
Source: Mystici Corporis Christi (Vatican.va)