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Replaced the U.S. Standard Industrial Classification (SIC) system in 1997; it's a 5- or 6-digit number. You can search by keyword, and there are tables to convert from NAICS to SIC, and SIC to NAICS. The NAICS was developed jointly by the U.S., Canada, and Mexico to compare statistics about business activity across North America.
The U.S. government classified business activity according to the SIC system until the adoption of NAICS in 1997. It's searchable by code or by keyword. Some business sources still feature SIC code search options.