I would actually say justice, although my understanding of that term is probably different from that of most leftists.
In Remnants of Auschwitz, Agamben says that law is not directed toward the establishment of justice, nor is it directed toward the verification of truth. Law is solely directed toward judgment, independent of truth and justice. According to Agamben, punishment is only important in that it is a fulfillment of the judgement already pronounced.
The widespread use of the term justice to refer to the outcome of juridical proceedings simply mistakes judgment for justice.
However, I don't think it is possible to bring about justice simply by skipping (or pretending to skip) over judgment and immediately meting out punishments (i.e. vengeance.) It seems to me that the concept of vengeance is almost inevitably a kind of mirror image of state judgement-punishment, differing only in that the question of judgment is hidden from view. I would say that a person enacting vengeance probably always passes judgement on the person they attack-- they just haven't given them a trial.
For me the nice thing about the notion of justice is actually its "ahistorical existing in a vacuum" status, as dot put it. This means that we can work out what it would mean to achieve justice not only in the sense that the state pretends to offer it or in merely parallel senses (such as attacking individual neo-nazis, or rapists, or politicians (not that doing those things is a bad idea per se)) but potentially in a much broader sense.
For me, a useful notion of justice would have to go beyond the simplistic ascription of guilt or innocence to individual people, and would show clearly that neither law nor vengeance can resolve our biggest problems.