Here's Sean's definition of what a growth hacker is:

"A growth hacker is a person whose true north is growth . Everything they do is scrutinized by its potential impact on scalable growth. The common characteristic seems to be an ability to take responsibility for growth and an entrepreneurial drive (it's risky taking that responsibility). An effective growth hacker also needs to be disciplined to follow a growth hacking 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."

Source: Sean Ellis

Sean Ellis is the Founder and CEO of GrowthHackers.com. He previously led Qualaroo through its acquisition and held early growth leadership at Dropbox, Lookout, Eventbrite, LogMeIn (launch to IPO), Uproar (launch to IPO). Sean is credited with having coined the phrase "growth hacker" in 2010.