My definition: A startup is a 1-5 year old company that has a potential to grow up its enterprise value by 50X within the next 5 years and has a business that can scale up quite easily.

This would filter out all the usual suspects Facebook, Twitter, 37 signals, while keeping Path, Instagram and all the YC funded companies. It would also cut out the new neighborhood bookstore, most service businesses and that sweet home business. Also it would cut out companies that have already reached $1-2bn in valuation (as they cannot realistically grow to become $100bn companies in 5 years).