Exclusive English translation of St. Alphonsus Liguori

What is the Sin of Usury?
The Church’s Doctor of Moral Theology explains

The sin of usury may very well be the most complex topic in all of Catholic moral theology. It involves a great many distinctions and nuances, and what adds to the difficulty is that usury is determined in part by the nature, function, and value of money, which has not always been the same throughout human history.

A pre-Vatican II Catholic dictionary defines “usury” succinctly as follows:

Usury is strictly speaking profit exacted on a loan of money just because it is a loan.