A small business is a business that is privately owned and operated, with a small number of employees and relatively low volume of sales. The legal definition of "small" often varies by country and industry, but is generally under 100 employees in the United States and under 50 employees in the European Union. In comparison, the definition of mid-sized business by the number of employees is generally under 500 in the U.S. and 250 for the European Union. In Australia, a small business is defined as 1-19 employees and a medium business as 20-200 employees. Small businesses are normally privately owned corporations, partnerships, or sole proprietorships.

In addition to number of employees, other methods used to classify small companies include annual sales (turnover), value of assets and net profit (balance sheet), alone or in a mixed definition. These criteria are followed by the European Union, for instance (headcount, turnover and balance sheet totals). Small businesses are usually not dominant in their field of operation.

Small businesses are common in many countries, depending on the economic system in operation. Typical examples include: convenience stores, other small shops (such as a bakery or delicatessen), hairdressers, tradesmen, lawyers, accountants, restaurants, guest houses, photographers, small-scale manufacturing etc.

The smallest businesses, often located in private homes, are called microbusinesses (term used by international organizations such as the World Bank and the International Finance Corporation) or SoHos. The term "mom and pop business" is a common colloquial expression for a single-family operated business with few (or no) employees other than the owners. When judged by the number of employees, the American and the European definitions are the same: under 10 employees.