Growth hacking is a process of rapid experimentation across marketing channels and product development to identify the most efficient ways to grow a business. Growth Hacking is more of Mindset than Toolset. Growth hacking  refers to a set of both conventional and unconventional marketing experiments that lead to growth of a business. Growth hackers are marketers, engineers and product managers that specifically focus on building and engaging the user base of a business.  Growth hackers often focus on low-cost alternatives to traditional marketing, e.g. using social media, viral marketing or targeted advertising  instead of buying advertising through more traditional media such as radio, newspaper, and television. 
Growth hacking is particularly prevalent with startups, when the goal is rapid growth at an early-stage launch phase.  Growth hacking may focus on lowering cost per customer acquisition, but growth hacking is a focus on long-term sustainability as Mason Pelt points out in a 2015 article on SiliconANGLE.com "The goal of any marketing should be long-term sustainable growth, not just a short-term gain. Growth hacking is about optimization as well as lead generation. Imagine your business is a bucket and your leads are water. You don't want to pour water into a leaky bucket; it's a waste of money. That's why a true growth hacker would care about customer retention." 
Those who specialize in growth hacking use various types of marketing and product iterations—rapidly testing persuasive copy, email marketing, SEO and viral strategies, among others, with a purpose to increase the conversion rate and achieve rapid growth of the user base. It can also involve on-line community management and social media outreach or highly personalized outreach to news outlets  to improve performance metrics such as driving customer acquisition and selling products.  Some consider growth hacking a part of the online marketing ecosystem  , as in many cases growth hackers are using techniques such as search engine optimization, website analytics, content marketing and A/B testing.
Product development is also heavily influenced by the growth hacker mindset. Instead of long development cycles followed by user testing. Growth hackers start user testing with wireframes and sketches; validating ideas at every stage. A growth hacker in a product development role would start user testing in a coffee shop instead of a corporate usability lab. 
Sean Ellis coined the term " growth hacker " in 2010.   In the blog post, he defined a growth hacker as " a person whose true north is growth. Everything they do is scrutinized by its potential impact on scalable growth. "  Andrew Chen introduced the term to a wider audience in a blog post titled, "Growth Hacker is the new VP Marketing" in which he defined the term and used the short term vacation rental platform Airbnb's integration of Craigslist as an example.  He wrote that growth hackers " are a hybrid of marketer and coder, one who looks at the traditional question of 'How do I get customers for my product?' and answers with A/B tests, landing pages, viral factor, email deliverability, and Open Graph. "   In 2012, Aaron Ginn defined a growth hacker on TechCrunch as a "mindset of data, creativity, and curiosity."  In the book "Growth Hacking", Chad Riddersen and Raymond Fong define a Growth Hacker as "a highly resourceful and creative marketer singularly focused on high leverage growth"
To combat this lack of money and experience, growth hackers approach marketing with a focus on innovation, scalability, and user connectivity.  Growth hacking does not, however, separate product design and product effectiveness from marketing.  Growth hackers build the product's potential growth, including user acquisition, on-boarding, monetization, retention, and virality, into the product itself.  Fast Company used Twitter " Suggested Users List " as example: "This was Twitter's real secret: It built marketing into the product rather than building infrastructure to do a lot of marketing."  However growth hacking isn't always free. TechCrunch shared several nearly free growth hacks  explaining that growth hacking is effective marketing and not mythical marketing pixie dust. As new tools(SaaS) come out specifically that focus on more advanced forms of Growth Hacking, more and more tools are being offered as free. 
The heart of growth hacking is the relentless focus on growth as the only metric that truly matters.  Mark Zuckerberg had this mindset while growing Facebook.  While the exact methods vary from company to company and from one industry to the next, the common denominator is always growth. Companies that have successfully "growth hacked" usually have a viral loop naturally built into their onboarding process.  New customers typically hear about the product or service through their network and by using the product or service, share it with their connections in turn. This loop of awareness, use, and sharing can result in exponential growth for the company. 
Twitter, Facebook, Dropbox, Pinterest, YouTube, Groupon, Udemy, Instagram and Google are all companies that used and still use growth hacking techniques to build brands and improve profits.   
An early example of " growth hacking " was Hotmail's inclusion of " PS I Love You " with a link for others to get the free online mail service.  Another example was the offer of more storage by Dropbox to users who referred their friends.  
Online worldwide independent lodging company, Airbnb, is an example of growth hacking by coupling technology and ingenuity. Airbnb realized they could essentially hack the Craiglist.org scale and tap both into their user base as well as their website by adding automated listing generators from Airbnb with the feature called "Post to Craigslist". The company's growth was a combination of clever thinking and technical know-how.
One of the easiest examples of " growth hacking " is to add "?sub_confirmation=1" at the end of your Youtube channel URL. Everybody will see a popup "Confirm Channel Subscription" (works only on desktops). This growth hacking tactic increases YouTube subscribers by 400%. 
Noah Kagan's submission form had four fields: Name, Email, URL, Revenue. He decided to remove the "revenue" field altogether, leaving only three fields—Name, Email and URL. This small change meant an improvement in his conversion rate of 26%. 
The University of Alberta increased email subscribers by 500% using a popup survey by Qualaroo that asked anyone who spent more than 10 seconds on the site: “You seem interested in UAlberta news. Would you like to sign up for the Daily News email?”.