"Growth hacking is merging engineering, product management, and marketing to grow a company's user base organically," tweeted Jon Miller, cofounder of Marketo.
Sean Ellis further says, "The common characteristic [for a successful growth hacker] seems to be an ability to take responsibility for growth and an entrepreneurial drive (it’s risky taking that responsibility). The right growth hacker will have a burning desire to connect your target market with your must have solution. They must have the creativity to figure out unique ways of driving growth in addition to testing/evolving the techniques proven by other companies. An effective growth hacker also needs to be disciplined to follow a process of prioritizing ideas (their own and others in the company), testing the ideas, and being analytical enough to know which tested growth drivers to keep and which ones to cut. The faster this process can be repeated, the more likely they'll find scalable, repeatable ways to grow the business."
This concept is obviously the foundation to free-to-play games. I saw work successfully at Com2uS (a leading mobile game company).
Other good articles on Growth Hackers: Andrew Chen (Growth Hacker is the new VP Marketing), and Aaran Ginn's, What is a Growth Hacker?