Unfairness is what results when a situation isn't equal or just. A teacher giving good grades to his favorite students is one example of unfairness.
When someone doesn't have to follow the rules, that's unfairness. You might accuse your parents of unfairness if they give your brother a larger allowance even though you do more chores. Unfairness can also be called injustice or inequality. This noun once had a completely different meaning, "ugliness," because fair can also mean beautiful.