What is a social justice warrior?

Continuing this post, the wiki on the term SJW is instructive. Originally it meant someone fighting for social justice. Only later was it used pejoratively to imply people so engaged did so only for personal validation instead of caring about injustice. It was a tactic to dismiss real social injustice.* Which is not to say that some who are so-called SJWs don't use social justice issues for dysfunctional purposes, but I'm not inclined to label every SJW because of that.

* Note the reference to the Gamergate controversy, where those who used the term specifically did so to denounce progressives but were "dismissed by commentators as trivial, conspiracy theories, groundless, or unrelated to actual issues of ethics." "Gamergate is often viewed as a right-wing backlash against progressivism."