That's one of my favourite misuses of language on HN.
A start-up is a thing that you depend on or plan to depend on for a substantial part of your income, preferably all of it.
It takes up more time than anything else you are doing because it is probably the most important thing in your life at the moment (with the exception of your nearest family).
It is not some web project that you threw together in an afternoon, though it can not be ruled out that what you threw together in an afternoon will gain some traction and will eventually reach the state where it will fit the above criteria.
A start-up is a thing that you depend on or plan to depend on for a substantial part of your income, preferably all of it.
It takes up more time than anything else you are doing because it is probably the most important thing in your life at the moment (with the exception of your nearest family).
It is not some web project that you threw together in an afternoon, though it can not be ruled out that what you threw together in an afternoon will gain some traction and will eventually reach the state where it will fit the above criteria.