Our restoring Principle, the Incarnate Word, being both God and human, instituted the medicine of the sacraments for the salvation of humankind in an orderly, distinct, and powerful way, in accord with the demands of his goodness, wisdom, and might. And so, when he entrusted this sacramental remedy to human beings, he willed that it be dispensed, not in a haphazard fashion, but in a way that would reflect that order, distinction, and power. Therefore it was appropriate that certain persons be distinguished and set apart to carry out this office, and that the requisite power be committed to them as a matter of ordinary law. Now a distinction of this kind could not be accomplished properly except by means of sacred signs such as the sacraments. Therefore, it was fitting that there be sacrament to be such a sign that would impart order, distinction, and power for the purpose of dispensing the other sacraments in a distinctive, effective, and orderly manner. That is why Orders is defined as "a certain sign whereby spiritual power is conferred upon the person ordained," for this definition contains the three elements that we have just mentioned.
[St. Bonaventure, Breviloquium, Monti, ed. and tr., Franciscan Institute Publications (Saint Bonaventure, NY: 2005), pp. 254-255. The definition is from Peter Lombard.]
The use of the plural term 'Orders' is a concession by the translators to common practice in English; Bonaventure, like all scholastic theologians, uses the singular, ordo, Order. The point is relevant because the term is used literally: it is the sacrament by which the Church is given an ordering, set in order, made orderly.