This definition is critical because it will determine one's understanding of this sin. All sin is a matter of life or death, whether it is a sin that people always talk about, or one that no one ever talks about, such as usury. All sin is serious in God's eyes, whether it is taken seriously by the Church or totally ignored by the Church.
The failure to have a right understanding of any sin leads to sin, not only because one did not know one has sinned. It is because having a wrong understanding of sin is itself manifestation of the flesh, which hates God.
For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit.
6 For to be carnally minded is death; but to be spiritually minded is life and peace.
7 Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be.
8 So then they that are in the flesh cannot please God.
9 But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.
10 And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness.
11 But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you.
12 Therefore, brethren, we are debtors, not to the flesh, to live after the flesh.
13 For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live.
Ezekiel 18:1-20 also says:
3 As I live, saith the Lord God, ye shall not have occasion any more to use this proverb in Israel.
4 Behold, all souls are mine; as the soul of the father, so also the soul of the son is mine: the soul that sinneth, it shall die.
5 But if a man be just, and do that which is lawful and right,
6 And hath not eaten upon the mountains, neither hath lifted up his eyes to the idols of the house of Israel, neither hath defiled his neighbour's wife, neither hath come near to a menstruous woman,
7 And hath not oppressed any, but hath restored to the debtor his pledge, hath spoiled none by violence, hath given his bread to the hungry, and hath covered the naked with a garment;
8 He that hath not given forth upon usury, neither hath taken any increase, that hath withdrawn his hand from iniquity, hath executed true judgment between man and man,
9 Hath walked in my statutes, and hath kept my judgments, to deal truly; he is just, he shall surely live, saith the Lord God.
10 If he beget a son that is a robber, a shedder of blood, and that doeth the like to any one of these things,
11 And that doeth not any of those duties, but even hath eaten upon the mountains, and defiled his neighbour's wife,
12 Hath oppressed the poor and needy, hath spoiled by violence, hath not restored the pledge, and hath lifted up his eyes to the idols, hath committed abomination,
13 Hath given forth upon usury, and hath taken increase: shall he then live? he shall not live: he hath done all these abominations; he shall surely die; his blood shall be upon him.
14 Now, lo, if he beget a son, that seeth all his father's sins which he hath done, and considereth, and doeth not such like,
15 That hath not eaten upon the mountains, neither hath lifted up his eyes to the idols of the house of Israel, hath not defiled his neighbour's wife,
16 Neither hath oppressed any, hath not withholden the pledge, neither hath spoiled by violence, but hath given his bread to the hungry, and hath covered the naked with a garment,
17 That hath taken off his hand from the poor, that hath not received usury nor increase, hath executed my judgments, hath walked in my statutes; he shall not die for the iniquity of his father, he shall surely live.
18 As for his father, because he cruelly oppressed, spoiled his brother by violence, and did that which is not good among his people, lo, even he shall die in his iniquity.
19 Yet say ye, Why? doth not the son bear the iniquity of the father? When the son hath done that which is lawful and right, and hath kept all my statutes, and hath done them, he shall surely live.
20 The soul that sinneth, it shall die. The son shall not bear the iniquity of the father, neither shall the father bear the iniquity of the son: the righteousness of the righteous shall be upon him, and the wickedness of the wicked shall be upon him.
Easton's Bible dictionary got it right when it said:
Usury [N] [S]
the sum paid for the use of money, hence interest; not, as in the modern sense, exorbitant interest. The Jews were forbidden to exact usury ( Leviticus 25:36 Leviticus 25:37 ), only, however, in their dealings with each other ( Deuteronomy 23:19 Deuteronomy 23:20 ). The violation of this law was viewed as a great crime ( Psalms 15:5 ; Proverbs 28:8 ; Jeremiah 15:10 ). After the Return, and later, this law was much neglected ( Nehemiah 5:7 Nehemiah 5:10 ). (emphasis mine)
Usury is a moral issue, and does has a true definition, which is determined by God. It is that usury is all interest, and that all usury is sin. To say that usury has no fixed definition is akin to saying that child sacrifice (called 'abortion') has no fixed definition, such that child sacrifice, and therefore its rightness or wrongness, is a matter of how one defines it. This leads to the logical conclusion that the usury is not absolutely right or wrong, but that its rightness or wrongness dependent on the circumstances. That is the danger of confusion about the definition of usury and why it is absolutely crucial for one to understand what usury truly is.
The Scholastic Analysis of Usury, John Noonan