Is there a Christian definition of "social justice?" Most (if not all) major Christian denominations seem to support the idea of "social justice." Most sources I've read seem to agree that the term itself was originally a Christian term that's gotten traction in more secular circles. Nowadays, people tend to use it mostly as a catch-all for a wide variety of political movements they believe in. I've seen "social justice" tied to campaigns for organic food, both sides of the abortion debate, tax policy, you name it.

My question is: Is there a specific Christian definition of the term "social justice" In particular, is it something different from plain ol' justice, or is the extra word just redundant?
No, there's not a specific Christian definition for this phrase.  It means various things to different people based on their own values.  The defiition, in general is the same as it is in the secular world - to champion social causes that are considered "Just".  Beyond that, it gets into "What's 'Just'?".

Put simply, there's no "Specific Christian definition" because "Christianity" is not one single view, it's the sum total of all the views of the believers.  See Is it valid to ask if “Christianity” teaches anything?