According to Collins English dictionary, a person who campaigns zealously and vociferously for causes associated with social justice.

In the derogatory sense, according to The Foundation for Economic Education, Know Your Meme identifies it as...” pejorative label applied to bloggers, activists and commentators who are prone to engage in lengthy and hostile debates against others on a range of issues concerning social injustice, identity politics and political correctness. In contrast to the social justice blogosphere at large, the stereotype of a social justice warrior is distinguished by the use of overzealous and self-righteous rhetorics [sic], as well as appealing to emotions over logic and reason.”