Roman Catechism 21.22

Chapter 21 — Part II: The Sacraments — Holy Orders

Section 22. In fact, order, when understood in its strict meaning and acceptation, is the arrangement of superior and inferior things so disposed as to stand in mutual relation towards each other. Now as in this ministry there are many grades and various functions, and as all these are disposed and arranged according to a definite plan, the name Order has been well and properly applied to it.