General Synonyms:
* Fairness
* Equity
* Rightfulness
* Righteousness
* Legality
* Lawfulness
* Impartiality
* Equality
* Due process
* Merit
More Specific Synonyms:
* Retributive Justice: Punishment, retribution, vengeance
* Distributive Justice: Fairness, equity, proportionality, meritocracy
* Restorative Justice: Reconciliation, healing, rehabilitation, forgiveness
* Procedural Justice: Due process, fairness, impartiality, transparency
Informal Synonyms:
* Right thing to do
* What's fair
* What's deserved
* What's right
The best synonym to use will depend on the context of your sentence and the specific meaning you want to convey.