You’ve asked about a very deep, complex subject with literally thousands of years of history, though, so the best I can do for you here is a very basic introduction.

First, let me recommend that you read my booklet Social Justice vs. Biblical Justice: How Good Intentions Undermine Justice and Gospel. It will give you a good foundation in a Biblically derived definition of justice.

Okay, that said, here’s a little background for you:

Harold Berman, in Law and Revolution: The Formation of the Western Legal Tradition, p. 11, wrote:

To speak of the Western legal tradition is to postulate a concept of law, not as a body of rules, but as a process, an enterprise, in which rules have meaning only in the context of institutions and procedures, values, and ways of thought. From this broader perspective, the sources of law include not only the will of the lawmaker but also the reason and conscience of the community and its customs and usages. This is not the prevailing view of law. But it is by no means unorthodox: it used to be said, and not long ago, that there are four sources of law: legislation, precedent, equity, and custom. In the formative era of the Western legal tradition there was not nearly so much legislation or so much precedent as there came to be in later centuries. The bulk of law was derived from custom, which was viewed in the light of equity (defined as reason and conscience). It is necessary to recognize that custom and equity are as much law as statutes and decisions, if the story of the Western legal tradition is to be followed and accepted.

Note that definition of equity. That was how it was understood for centuries and was seen to be almost synonymous with justice, fairness, each of which basically meant the impartial and proportional application of the fundamental law/laws of God (whether expressed as the laws of nature or as the Ten Commandments or some other compendium) to all people in all circumstances. Over time equity grew to include the idea of the lessons learned over time by the efforts to achieve that application of fundamental law to specific people and circumstances—which amounted to precedent. This yielded, in Anglo-American law, the development of “courts of equity,” or sometimes a court was itself be called simply “equity”—in which disputes were settled not on the basis of statute (usually because no statute addressed the specific situation) but on the basis of equity. (See https://thelawdictionary.org/equity/ and https://dictionary.law.com/Default.aspx?selected=646. And in looking those up, I just came upon https://wmmlegal.com/divorce-law-equity-vs-equality#:~:text=Equity%2C%20per%20Black’s%20Law%20dictionary,of%20making%20things%20%E2%80%9Cright%E2%80%9D., which helpfully distinguishes equity from equality.)

Largely as an outworking of Marxist thought, over the last century or so, little by little but in the last decade or so in a torrent, equity has come to be taken as synonymous with equality—whether of opportunity or outcome. The practical result is to turn real equity on its head, since, because people differ hugely in myriad ways, the only way to ensure equality among them is precisely to treat them differently, to be partial, to be unequal in the application of basic moral principle—it is to treat them unjustly. That is the sense of it in the Progressive movement and especially the Woke movement. The result is to fuel envy and resentment on the part of anyone who is in any way (detrimentally) unequal to anyone else—i.e., to declare covetousness (violating the 10^th Commandment) right and good.

For further study, see:

• Voddie T. Baucham, Jr., Fault Lines: The Social Justice Movement and Evangelicalism’s Looming Catastrophe
• Owen Strachan, Christianity and Wokeness
• Scott David Allen, Why Social Justice Is Not Biblical Justice: An Urgent Appeal to Fellow Christians in a Time of Social Crisis

I hope this is helpful, and I commend you for wanting to learn much more about this.