In current usage, a startup means a new business that is designed to scale rapidly. (Most don't actually manage to, but they're at least intended to.) It would be inconvenient if people started using the word to describe new businesses generally, because then we'd need to invent a new word for the subset designed to scale rapidly.
That's about as precise as most dictionary definitions. It doesn't mean the actual meaning of the word is that broad, just that dictionaries don't go into excessive detail.
There seems to be varying opinions on what a startup actually is. Some definitions focus on the ability to rapidly scale, while others emphasize the fragility of startups. For instance, Eric Ries, author of The Lean Startup, defines a startup as "a human institution designed to deliver a new product or service under conditions of extreme uncertainty." This definition would tend to include almost every new business, while your definition of a startup would only include those which are structured for rapid growth. Maybe I'm being a bit too pedantic, but it seems the word "startup" is being used so inconsistently nowadays that it has lost a bit of its meaning.
PG I actually thought about my article heading a lot and went back and fort between lifestyle "startups, businesses and entrepreneurs" for the right word to describe my post. At the end I decided on startups since the other two words are further off what I intended to convey. But I definitely can see your point and how it can become confusing.
