This article's factual accuracy is disputed. (May 2017)
A scaleup company or just scaleup is defined by the OECD as a company having an average annualized return of at least 20% in the past 3 years with at least 10 employees in the beginning of the period.
The importance of scaleups and the rise of their terminology according to the World Economic Forum is that, although not all startups make it big, the ones that do greatly impact society by means of new technology, services and increased employment. 
To aid this rise, instead of large startup incubators, policy makers are more and more focusing on scaleups since they are the ones that add value.
One commonly used definition of 'scaleup' is the OECD-Eurostat definition relating to 'gazelles' or High Growth Firms: "All enterprises with average annualised growth greater than 20% per annum, over a three year period should be considered as high-growth enterprises. Growth can be measured by the number of employees or by turnover."
Evolution of a scaleup
One way of looking at the evolution of a startup into a scaleup is that scaleups evolve from startups as they cross the (so called) "growth chasm"  that is, once they solve the startup challenges of market research, development, and identifying a repeatable, scalable business model. This can be identified by a significant sustained repetition of critical mass in a particular startup's most significant metric – generally, this metric is revenue, number of employers, number of active users, number of active customers, or effective reach, relative to funds raised.
A key difference between a startup and a scaleup is the main challenges faced. While a startup's main challenge is to find a repeatable scalable business model, a scaleup's main challenge is growth of the already identified business model while maintaining operational controls.
A 2018 study showed that only 0.4 percent of all startups scale, reaching more than 10 million revenues within 5 years. The remainder stabilizes or grows at a much lower rate.
Economic importance of Scaleups
Endeavor published a study reporting that scaleups create most of Southeast Asia's new jobs. Endeavor founder/CEO Linda Rottenberg published the scaleup petition to further elevate the movement around scaleups.
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