The best answer I've seen ..

 •  Filed under Links, Startups

.. to defining what a start-up is.

A start-up should be viewed as continually evolving entity, not a static product or service. Entrepreneurs should use Lean to learn from customers and iterate accordingly. Decisions should be made based on data from customers, not purely a vision.

You Don’t Have to Be a Product Visionary to Create a Great Product by Huffington Post