How is "small business" really defined?

We hear the term thrown around all of the time in the media by politicians and others. When I think small business, I am thinking small corner store, or local business in the community. But, a friend of mine told me that the definition is actually for much bigger business than I think. The problem is she could not give me specifics. How many employees? How much revenue? How much profit?

What is the technical definition of small business when we talk about it referring to jobs and our economy?