injustice: a wrong done against what is right, especially when fairness or honest dealing is owed
Avel is the opposite of justice. It names a wrong, but not just any wrong: it points to a violation of what is fair and right, the kind of act that cheats, distorts, or treats someone unjustly. Hebrew writes this word two ways, a shorter masculine form and a longer feminine form, but they are spellings of one word with one meaning, and the writers use them interchangeably.
The word lives in three settings that all turn on the same idea. It can mean unfairness in dealing and judging: a crooked verdict, a rigged scale, a bribe that bends the law. It can broaden to wrongdoing in general, the wickedness of a person's deeds and words, with the courtroom edge softened into plain moral evil. And it appears again and again in a single emphatic claim about God, that with him there is no injustice at all.
What ties these together is not three separate ideas but one concept seen at different scales. Cheating a neighbor in the market and the broad wickedness of a violent generation are both injustice, the first close-up and the second wide-angle. And precisely because injustice is what avel means, denying any of it in God is the strongest way to say that he is perfectly fair.