Wikipedia says a startup is "a partnership or temporary organization designed to search for a repeatable and scalable business model"[1]. The relevant Wiktionary entry defines it as a "new organization or business venture"[2], fairly similar to the OED's "newly established business" definition[3].
[1] https://en.wikipedia.org/wiki/Startup_company
[2] http://en.wiktionary.org/wiki/startup
[3] http://www.oxforddictionaries.com/us/definition/american_eng...