" A growth hacker is someone who has thrown out the playbook of traditional marketing and replaced it with only what is testable, trackable, and scalable. Their tools are e-mails, pay-per-click ads, blogs, and platform APIs instead of commercials, publicity, and money. While their marketing brethren chase vague notions like “branding” and “mind share,” growth hackers relentlessly pursue users and growth—and when they do it right, those users beget more users, who beget more users. They are the inventors, operators, and mechanics of their own self-sustaining and self-propagating growth machine that can take a start-up from nothing to something. "

― Ryan Holiday , Growth Hacker Marketing: A Primer on the Future of PR, Marketing, and Advertising