I think it's just as simple as a company that can grow really fast, usually backed by some form of venture capital. Otherwise you'd be saying a simple product/s

I think it's just as simple as a company that can grow really fast, usually backed by some form of venture capital. Otherwise you'd be saying a simple product/service bysiness is a startup, since someone selling a forum script or web hosting is also 'doing the work once and selling it multiple times'.