What is the definition of a business description on taxes?

It depends. I'm not sure if you are referring to a specific tax form, but for instance a Schedule C asks you to describe the activity or business that generated the income. Here you would describe the business in a broad sense, like real estate agent, content creator, electrical contractor, or whatever it is that you do.

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Is it asking for the Business Code?  The Business Code is a 6 digit number for your activity. Here are the IRS instructions for Schedule C. Business codes start on page 18.
http://www.irs.gov/pub/irs-pdf/i1040sc.pdf