By definition, a startup is any newly established company. We can argue over how much time defines what is newly established, but can't argue that the size, revenue, or money raised define what is new.
The definition of a startup is a company that is still searching for a viable business model / target customers / distribution channel. The size, age, amount raised or even revenue doesn't matter. Only that whatever revenue it has, it is sustainable, and customer acquisition is repeatable.