As I understand it, "growth hacking" simply refers to the practice of treating your company's marketing as a software project; of systemizing it, making it measurable and repeatable.
This is one of the reasons I am not in love with that word. For better or worse, for at least some people it is strongly associated with "naughtiness" in the sense talked about here: http://www.paulgraham.com/founders.html
At least some growth hacking tactics are examples of taking someone else's system and convincing it to do things that its makers had not envisioned. The paradigmatic example was Airbnb building a deep Craigslist integration via a process which does not wholly fail to resemble security research. http://andrewchen.co/2012/04/27/how-to-be-a-growth-hacker-an...