What growth hacking really is

Growth hacking are two words that I keep hearing since I started working at Social Media and Marketing, they are everywhere, and I even saw a lot of people looking for growth hackers to design and run their campaigns.
However, altough the words are quite known by everyone, it really took me some time to find out what growth hacking really is, most of people just described it as a clever technique that generates more users and followers or something like that.
I put together some concepts regarding what Growth Hacking really is:

Growth hacking isnt the best term, but it describes a new process for acquiring and engaging users combining traditional marketing and analytical skills with product development skills. In the past, marketing and product development departments were often at odds where marketing groups would be spending significant amounts of money to acquire users but couldnt get any development resources to build something as simple as new custom landing pages.
The founding and growing process of Twitter is an interesting example
Many many users were hearing about Twitter each day from press, blogs, their friends and were signing up. But none of them stuck around. Typical marketing efforts in the past would have been to use email newsletters to bring users back, or spend money on display retargeting. But instead they invested in the product. They made some research and tried to learn what the precise moment was for a new user and then rebuilt their entire new user experience to engineer that more quickly. It turned out that if you manually selected and followed at least 5-10 Twitter accounts in your first day on Twitter, you were much more likely to become a long term user, since you had chosen things that interested you. And if we helped someone you know follow you back, then even better. This I believe is an excellent experience of growth hacking.
However there is no magic formula for growth hacking, only experiences, some of them were sucesfull, many of them were big failures.
Any attempts at artificially creating growth patterns such as spamming friends on Facebook or Twitter, may result in spiky numbers, but rarely adds retained users. Its kind of like eating empty calories. Instead, you have to search through your data from your more active and passionate users and discover the deep core patterns that encouraged those users to become active. And then you have to build sustainable features that help attract users on a continuous basis such as good viral flows or great SEO landing pages, and then an experience that helps users quickly understand and become active within your product.
I think the best part of the current hype around growth hacking is that it helps remind everyone how important it is to grow your product. You need to learn from your data and understand what innately in your product helps your active users attract new users, and then create solutions that help engineer that to happen more quickly and more often. Knowing how to build and market great products have always been core attributes of successful companies.
As I said at the beginning, my experience in Social Media is quite fresh, and in growth hacking is even fresher, im still making my first steps at this industry, so these were just some few thoughts I have in my head.
What do you think? how would you accurately describe growth hacking?