Accelerator

I think the best explanation for an accelerator is the following:
business accelerator  is an intensive (usually 3 months), business program which includes mentorship, educational components, networking and aims at growing business rapidly, ending in demo-day. Usually an entrepreneur moves into a shared office space with other new founders for a period of time to work under the tutelage of advisors and experts to grow their business rapidly. In exchange for the expert mentoring, exposure to investors/future capital and cash investment that entrepreneurs get from the accelerator, the entrepreneur gives a portion of his or her company’s equity to the partners of the program and for this reason is often called a “seed” or “venture” accelerator. ( Vasily Ryzhonkov)

Here are some similar definitions that might be useful

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The Startup Factories Report  by Paul Miller and Kirsten Bound (2011):  The  accelerator program model comprises five main features which set it apart from other approaches to investment or business incubation: ======

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Wikipedia:  Seed accelerators  are fixed-term, cohort-based programs, that include mentorship and educational components and culminate in a public pitch event or demo-day.[1] While traditional business incubators are often government-funded, generally take no equity, and focus on biotech, medical technology, clean tech or product-centric companies, accelerators can be either privately or publicly funded and focus on a wide range of industries. ======

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U.S. Department of Commerce Economic Development Administration (Lewis et al. (2011):  Business accelerator  is (1) a late-stage incubation program, assisting entrepreneurial firms that are more mature and ready for external financing; or (2) a facility that houses a modified business incubation program designed for incubator graduates as they ease into the market. ====== An interesting article that explains the arise of accelerators is:

http://worldbusinessincubation.wordpress.com/2014/03/12/startup-accelerators-the-history-and-definition/