It’s whatever the liberal group using the term wants it to mean to push their agenda.
But normally can be translated as, "You BAD! Me GOOD!! Now pay up!!"
That's true - the specific expression of "social justice" can change, but this is it's essence.
Ding, ding, ding! We have a winnah!
Like Truth (that which serves the Party the best), Social Justice, or fairness, is that policy, that outcome, that favors me the most (and my social class, to put it into Bolshevik terms).
Thus, both fairness and truth come with a built-in sliding scale. There is no objective standard because they keep moving the goalposts.
Thus, they are able to say that civil rights progress from the fifties, to the sixties, to the seventies, etc, has been virtually nil; and that the election of a colorful man to the White House proves nothing about our being newly-tolerant, either.
It's just a little bit frustrating, so it pays to know that it's because of a total lack of objectivity on the part of our opponents, not because of any objective malfeasance or misanthropy on our part. However, I must admit, their efforts to make racists out of us are starting to bear some fruit. And don't get me started on the fruit thing.