Ask HN: When do you call it a startup?

I realize this is more of a question on semantics, but it usually leads to some interesting discussion.

At what point does 'it' become a startup? Is it when you incorporate, get funding, or some other milestone? When you drop everything else and work on it exclusively? When you get your first users?

There are so many goals and tasks of a startup that I've always wondered what others define as the 'beginning' of a startup. What're your thoughts?