What is the definition of small businesses in USA?

I’ve always heard it was under 100 employees.

Welcome to Fluther.

It’s not so much a matter of “number of employees”, because some seasonal businesses such as ski resorts, farms and other spots that don’t have permanent employees can still hire employees in the hundreds—for weeks or months at a time—and are still obviously “small” businesses. And it’s not based on income, either, or some individuals would classify as “big” business, and even if they are in business, they themselves are not “big business”.

So I have a rule of thumb that I’ve been thinking of just to answer this question:
1. Big Business can afford its own Senator.
2. Sort-of Big Business can only afford a Representative.
3. A business that can only afford its own (dedicated) lobbyist is a Medium Business.
4. One that can only afford a “lobbying organization” (or has to join some sort of federation or organization to gain representation) is a Small Business.
5. If you can’t afford a lobbyist at all, then what you have would be considered a hobby.

The Small Business Administration has determined that small businesses are classified by industry type. Their chart is comprehensive.