Hacking, in this context, is not illegal! Hacking refers to the process of getting things done, regardless of the process. Growth hacking is a term used in the marketing world and it means implementing processes and tools to improve user reach and consumption in marketing campaigns. This could be achieved using computer algorithms, tools and logic resources.
For example…
Airbnb’s growth hacking attempt is a simple yet effective example. When hosts listed their property on the Airbnb website through a form, they presented an option for the user to post their listing on Craigslist as well. This presented Airbnb with another avenue through which to advertise their services. The backlink provided on the popular Craigslist website meant they increased their rankings in search engines as well.